What Is a MAB File? The Address Book Thunderbird Left Behind
A MAB file holds contacts. Jump to your question below, or work through it in order.
A MAB file holds contacts. Jump to your question below, or work through it in order.
A MAB file is a Mozilla Address Book. Thunderbird, SeaMonkey and the old Netscape mail programs kept contacts in one: names, email addresses, phone numbers, street addresses and notes. All three are open source applications, and every one of them wrote the same format.
Inside sits a Mork database. Mork is text, so MAB files open in Notepad and you can read the contact fields yourself.
Nothing current writes one. Thunderbird moved its address book to SQLite on 17 July 2020, which is why the file you have will not open.
MAB is short for Mozilla Address Book. The extension marks a contacts database used by Mozilla mail programs for twenty-one years. A MAB file contains contact information and nothing else. Open one and you find people: names, email addresses, phone numbers, companies, street addresses, notes and the mailing lists those people belong to.
What makes the MAB file format interesting is the layer underneath. The contacts sit in a database format called Mork, and Mork is text. Not text with binary bolted on. Text from first byte to last, which means text editors read every part of your file.
Several guides say otherwise. One calls MAB files binary, with media type application/octet-stream. The old version of this page put Format Type: Binary in its specification table. Both are wrong, and it matters, because readers who believe their file is binary go looking for software instead of double-clicking it open.
Thunderbird wrote its last MAB file in 2020. That single fact reorders everything. You are not asking how to work in this format. You are asking how to get your contacts back out of something nothing opens.
.mab// <!-- <mdb:mork:z v="1.4"/> -->abook.sqlite, then vCard inside ittext/plainThunderbird 78 arrived on 17 July 2020 and converted every address book to SQLite. Its release notes say so in one sentence. From that day nothing in the product wrote one again.
Mozilla put it plainly in the Thunderbird 78.0 release notes. Address books are now stored as SQLite databases, and existing address books in MAB format, using a Mork database, will be converted.
That ended a long run. Mork came out of Netscape in December 1998. Netscape 6, the Mozilla Suite, SeaMonkey and every Thunderbird from 1.0 in December 2004 through the 68 series in 2019 stored contacts in it. Twenty-one years on one format.
Replacement work landed on the beta channel first, in Thunderbird 71, which switched the default address book from Mork to SQLite. Another change migrated books people already had. Then Thunderbird 102 in June 2022 moved those contacts to vCard, the same standard your phone uses.
| When | What changed | The file |
|---|---|---|
| Dec 1998 | David McCusker dates the Mork grammar at Netscape | Mork arrives |
| Dec 2004 | Thunderbird 1.0 ships, already using Mork | abook.mab |
| 2011 | Firefox drops Mork for history and bookmarks | Mail keeps it |
| Aug 2019 | Thunderbird 68, the last series to use MAB | abook.mab |
| 2019 beta | Thunderbird 71 switches the default to a SQLite book | abook.sqlite |
| 17 Jul 2020 | Thunderbird 78 converts every existing MAB file | abook.sqlite |
| 2021 | Thunderbird 91 adds CardDAV address books | Server-side |
| Jun 2022 | Thunderbird 102 stores contacts as vCard inside the database | abook.sqlite |
Mork is not gone from Thunderbird. The address book left, and the mail folder summary files did not. Every .msf file sitting beside your mail folders today is still a Mork database, written by the same code that used to write your contacts.
Two faults produce almost every MAB complaint. Nothing on the machine reads the format any more, or Thunderbird reads it fine and ignores your file because of what the file is called.
You double-click, and Windows asks which program to use. Nothing in the list works. The instinct is that your file arrived damaged. It almost never did, and a file that will not open is not damaged by default.
The first reason is simple. Current Thunderbird does not read the MAB file format, and no operating system ever shipped anything that did. Windows, macOS and Linux all treat .mab as an unknown extension.
The second reason catches more people, and it is the one nearly every guide leaves out. Thunderbird only ever loaded two file names by itself. Drop impab.mab or abook-2.mab into your profile folder and Thunderbird ignores both, because it looks for its own two names and nothing else.
| File name | What it held | Loads by name |
|---|---|---|
abook.mab | Personal Address Book, the contacts you typed in | Yes |
history.mab | Collected Addresses, gathered from mail you sent | Yes |
impab.mab | An imported address book | No |
impab-1.mab | A second imported book | No |
abook-1.mab | An extra book made inside Thunderbird | No |
abook-2.mab | Another extra book, and so on upward | No |
Archive Team's file format wiki is the only reference that states the rule outright. Copying files of that format into the directory will not work properly, because only two named files are found automatically. Everything else is known to Thunderbird only if Thunderbird created or imported it.
So the fix for an ignored file is a rename, not a repair. Back up whatever abook.mab is there now, rename yours to abook.mab and restart. Section 5 is the full sequence.
| What you see | What it often means | Start with |
|---|---|---|
| Windows asks which program to use | Nothing current reads the format | A text editor, then section 5 |
| Thunderbird ignores the file | Wrong file name | Rename it abook.mab |
| Contacts vanished after an update | Thunderbird 78 converted them already | Look for abook.sqlite |
| Walls of punctuation in Notepad | Working exactly as intended | Nothing. That is Mork. See section 8 |
| Only some contacts arrived | Only one of several books was loaded | The file name rule above |
| Unreadable bytes, and it is huge | Not a Mozilla file at all | Read section 10 |
Start with a text editor, because the file is text and you already own one. Then choose a route by what you want: a look, a conversion or the contacts back inside a mail program.
Every guide on the first page of results sells a converter. You may not need one. MAB files are text, and that puts a free answer on every computer already.
| Software | Runs on | Why choose it |
|---|---|---|
| Any text editor | Everywhere | Start here. Notepad, TextEdit, nano or gedit shows every contact |
| Thunderbird 78 | Windows, Mac, Linux | The answer. Converts your file, then exports to CSV or vCard |
| SeaMonkey | Windows, Mac, Linux | Its mail side still carries the older code, so it reads MAB directly |
| mork-converter | Anywhere Python runs | Writes XML, or CSV with one switch. Needs Python 2, which is the catch |
| mork-parser | Anywhere Node runs | Turns Mork into JSON, and it is short enough to read |
| LibreOffice | Windows, Mac, Linux | Its address book wizard was built for Mork rather than for the newer file |
The Python note on mork-converter matters more than it looks. That tool wants Python 2.4 or later, and Python 2 reached its end in 2020. On machines bought since then you would have to install an old interpreter before the converter runs at all.
Use any text editor. Nothing to install, nothing to buy. The contacts show up in about ten seconds.
Use Thunderbird 78. It converts and exports to CSV or vCard. Section 5 is the sequence.
LibreOffice is the odd one here, and the reason is worth a paragraph. Its address book connector was written for Mork, so it aims at the format Thunderbird dropped rather than the one Thunderbird uses. Apache OpenOffice shares that code, and carries the same wizard under the name OpenOffice.org used.
When Thunderbird moved to SQLite, The Document Foundation opened one bug whose title says it all: Thunderbird Address Book will be changed to sqlite, development needed.
Two of the four directories that rank for this file tell readers LibreOffice imports MAB. That was true of the builds they were written against. Whether it holds in the build on your machine varies, so treat it as worth a try rather than as the plan. Converting to CSV first works everywhere.
Do not upload contact files to a browser converter you do not know. Your address book is every colleague, client and family member you have. Text editors and Thunderbird 78 keep all of it on your own machine.
Five steps, in this order. The rename in step two is the one people skip, and skipping it is why Thunderbird appears to ignore a perfectly good file.
The aim is not to keep working in the MAB file format. It is to leave it. CSV and vCard both import into Outlook, Gmail, Apple Contacts, current Thunderbird and every phone, so either ends the problem for good.
abook.mab. Thunderbird loads that name and history.mab by itself and no others. An impab.mab sitting beside it stays invisible.abook.mab aside first, so nothing is overwritten. Section 6 gives the path on every system.abook.sqlite on first run, with no progress bar. Large books took minutes in the early builds, so give it time before deciding it failed.Do not edit a MAB file in a text editor and save it. Reading is safe. Writing is not, because every column name is interned once and then referenced by hex id, and edits that shift those ids leave the file parsing to nonsense.
Mozilla keeps old builds on its own archive server, so the version that does the conversion is still available. Install it into its own folder rather than over your current Thunderbird, point it at the profile holding your renamed file and remove it once the export is done.
If your current Thunderbird finds the file and converts it without any of that, take the win and skip ahead to the export. The migration code was written to run on any profile that still held Mork, so it is worth trying first.
Export vCard. Apple Contacts, Android and Gmail all take .vcf files straight in.
Export CSV. Outlook's import wizard reads it, and the column names are yours to fix first.
Import problems? vCards that arrive garbled or refuse to import usually carry an encoding fault rather than a bad export. Our guides cover garbled vCard names and the file is not formatted for contacts import error.
MAB files live inside a Thunderbird profile folder, never loose in Documents. The quickest way to that folder is from inside Thunderbird itself, not by typing a path.
Thunderbird profiles hold your mail, your settings and your address book together in one folder with a random name. MAB files sit inside that folder, not in the folder above it. The old version of this page stopped one level short and sent readers to Profiles itself, where there is nothing to find.
| System | Path |
|---|---|
| Windows | C:\Users\<your login name>\AppData\Roaming\Thunderbird\Profiles\ |
| macOS | ~/Library/Thunderbird/Profiles/ |
| Linux | ~/.thunderbird/, and on older installs ~/.mozilla/Thunderbird/ |
Inside that folder sits at least one profile, named something like 8f3k2p1q.default-release. Your MAB files are in there, beside prefs.js and the mail folders. AppData is hidden on Windows and ~/Library is hidden on a Mac, so paste the path rather than clicking through.
Close Thunderbird before you move anything. It holds the address book open while it runs, and files copied out from under a running program arrive half written.
| File | What it is |
|---|---|
abook.mab | The Personal Address Book, if the profile predates 2020 |
abook.sqlite | The address book after Thunderbird 78 converted it |
*.msf | Mail folder summaries, still Mork to this day |
prefs.js | Settings, including which address books exist |
Mail, ImapMail | Messages, stored as MBOX files |
*.bak | A backup copy Thunderbird kept of an address book |
Seeing both abook.mab and abook.sqlite in one profile is normal, and it is good news. It means the conversion already ran and your old file was left in place rather than deleted. The SQLite file holds your current contacts, and the MAB file is your copy from before the change.
Contacts that vanish, double up or come back after you delete them point at the address book, not at your account. The repair is a rename and a restart, and it has to happen with Thunderbird closed.
Four complaints turn up together often enough to be one fault. The address book opens empty. Contacts that were there yesterday are gone. Names appear twice. Deleted contacts come back at the next restart.
All four say one thing: your address book is not being written and read cleanly. That follows a crash during a save, a profile copied while Thunderbird was running or a disk that filled up mid-write.
Thunderbird kept a backup, and almost nobody knows. Look in the profile folder for a file ending .bak beside the address book. That is the previous copy, and swapping it in is the entire repair.
abook.mab to abook.mab.old. Renaming keeps it. Deleting does not.abook.bak to abook.mab. Windows hides known extensions by default, so turn that setting off in File Explorer first or the rename will not take.The older version of this page gave step four and nothing else. It told readers to unhide file extensions and remove .bak, with no mention of closing Thunderbird and no mention of moving the existing address book aside. Followed as written, that overwrites the file you were trying to save.
Duplicate contacts usually mean two address books, not one damaged book. Imports that ran twice leave impab.mab and impab-1.mab side by side, and Collected Addresses quietly adds anyone you have written to. Check which book every copy sits in before deleting anything.
Contacts that reappear after deletion point the other way. Something puts your old file back: a sync tool, a backup agent or a second profile you forgot about. Find what writes to that folder before fighting the symptom.
Open a MAB file in a text editor. Line one names the format, a dictionary block interns every field name against a hex code, and rows below spend those codes rather than repeating words.
David McCusker designed Mork at Netscape and dated the formal grammar 18 December 1998. His primer, from March 1999, states in as many words what it was built for. Collections of address book cards, where every card carries some subset of an open-ended set of attributes.
That requirement explains the design. Contacts have wildly different fields filled in, so fixed tables waste space. Mork stores a name once, gives it a short code, then writes that code. Disk space mattered in 1998 and this saved a lot of it.
Read that as three nested disguises. An XML tag, wrapped in an HTML comment, sitting behind what a C compiler would treat as a comment. Mork's grammar spells that production out with version 1.1, and real Thunderbird address books carry 1.4.
It is also the fastest test you own. Open your file, look at line one. That line means Mozilla Address Book. Anything else means your file holds something other than contacts.
| Mark | What it opens | What sits inside |
|---|---|---|
// <!-- | The magic line | The format name and version, once, at the top |
< > | A dictionary | Field names paired with the hex codes that stand for them |
{ } | A table | What kind of address book this is, and which rows belong to it |
[ ] | A row | One contact, written as hex codes and values |
^ | A reference | Spend a code defined earlier rather than repeating the word |
@$$ | A group | A batch of changes appended to the end of the file |
Codes from 80 upward stand for strings the file defines. Codes below that stand for the single character of that value, so the format never has to write a one-letter name twice. Strings you will meet in a real file include LastModifiedDate, RecordKey, AddrCharSet and ListName, plus the character set, which is normally iso-8859-1.
Two names say what the file is for. The table is marked ns:addrbk:db:table:kind:pab, meaning a personal address book. Its rows are marked ns:addrbk:db:row:scope:card:all, meaning contact cards.
Mork earned a reputation. Jamie Zawinski, who worked on the Netscape mail code, called it the single most braindamaged file format he had seen. Archive Team's wiki notes it is disowned even by its creator. Reading one is fine. Writing a correct parser for one is the hard part.
Mork appends. Changing a contact writes a new group at the end rather than rewriting that row in place, which makes saving fast and makes files grow. A book you have edited for years holds a long tail of superseded values, and that is normal rather than damage.
It also explains the sizes. A MAB file that runs from 1 KB to about 160 KB, with most landing near 28 KB, is a normal one. A .mab measured in megabytes is worth a second look, and section 10 covers the likely reason.
The file that replaced MAB is abook.sqlite. It is a real database rather than a text log, and since 2022 the contacts inside it are stored as vCard, which every other program already understands.
Comparing them says more about why this format was retired than any history can. Mork was built for machines with slow disks and little memory, and both problems went away.
| MAB | abook.sqlite | |
|---|---|---|
| Arrived | 1998 | 2020 |
| Retired | 17 July 2020 | Current |
| Encoding | Text you can read | Binary database |
| Underlying format | Mork, a Mozilla invention | SQLite, used everywhere |
| Contact fields | Mozilla's own field names | vCard, from Thunderbird 102 |
| Tools that read it | A handful, mostly abandoned | Any SQLite browser |
| Still supported | No | Yes, and it supports vCard |
| Edits | Appended, so the file grows | Written in place |
| Written by anything today | No | Yes |
Under the surface Thunderbird now reaches a local address book through a scheme called jsaddrbook, a CardDAV one through jscarddav and a company directory through LDAP. All three sit beside one another, and only the first keeps a file in your profile.
Oddly, the older format is the readable one. MAB files open in Notepad and a person can follow them. Their replacement needs a database tool, even though contacts inside sit in a standard anyone can parse. Readable by a human and readable by software are not the same property.
Convert it now. Every year makes the tools harder to find. Section 5 is the route.
Nothing to do. That is the current format, and Thunderbird keeps it up to date on its own.
Moving between mail programs? MBOX files carry the messages Thunderbird stores, PST files hold an Outlook mailbox and OAB files are Outlook's own offline address book, which people confuse with this one.
Not every .mab file is a Mozilla Address Book. SKF Machine Analyst, which reads vibration data from bearings, writes a different format under the same three letters.
A reader who arrived here with a machine maintenance file needs to know in ten seconds rather than nine sections in. The test is the first line, and the two formats could not look less alike.
| Mozilla Address Book | SKF Machine Analyst | |
|---|---|---|
| Starts with | // <!-- | Bytes 00 00 00 00 36 |
| In a text editor | Readable field names | Unreadable |
| Typical size | 1 KB to 160 KB | 160 KB to 23 MB |
| Named | abook.mab, history.mab | bearings.mab, export.mab |
| Holds | Contacts | Machine vibration measurements |
| Read it with | A text editor, Thunderbird 78 | SKF's own software |
Nothing on this page applies to the second kind. Renaming one to abook.mab and feeding it to Thunderbird achieves nothing, and no contact converter will read it either.
A third possibility is that the extension means nothing at all. Any program can write .mab, and some do. If the first line matches neither pattern above, the extension is telling you less than the file is.
Keep the MAB file, and stop depending on it. Export the contacts to vCard or CSV today, then archive the original. The file costs nothing to store and the tools that read it keep getting scarcer.
Storing a MAB file and relying on one are different decisions, and the answers run opposite ways. Keep it, because an original is always worth more than a copy and a contacts database is measured in kilobytes. Stop relying on it, because it depends on software that gets harder to find every year.
Think about what already happened. Thunderbird 78 landed in 2020. The mork-converter needs a version of Python that modern systems no longer ship. LibreOffice has an open bug about the format change rather than a finished answer. None of that improves with waiting.
Archive it. Kilobytes, and the only complete copy of what was in the book.
Export today. vCard for phones and Apple Contacts, CSV for Outlook and spreadsheets.
| Export as | Best for | Watch out for |
|---|---|---|
| vCard | Phones, Apple Contacts, Gmail, current Thunderbird | Accented names, if the encoding is wrong |
| CSV | Outlook, spreadsheets, anything you want to edit | Column names, which rarely line up first time |
| LDIF | Directory servers and older mail programs | Fewer programs read it every year |
| Leave as MAB | Nothing, once you have the export | The tools, which keep disappearing |
Both common export formats can be imported anywhere, so export twice if the contacts matter. A vCard file for the phone and a CSV for the spreadsheet cost one extra minute, and between them they survive anything.
Everything on this page in one block: names, paths, the line that identifies this format and the dates that ended it.
Directories file this under the MAB file type, and the file type is what decides your answer. The file extension MAB file managers show has meant one thing since 1998: the Mozilla Address Book is what wrote it.
| Item | Value |
|---|---|
| Extension | .mab |
| Full name | Mozilla Address Book |
| Database format | Mork |
| Designed by | David McCusker, at Netscape |
| Grammar dated | 18 December 1998 |
| Primer dated | 16 March 1999 |
| Magic line | // <!-- <mdb:mork:z v="1.4"/> --> |
| Encoding | Text, normally iso-8859-1 |
| Media type | None registered with IANA. Behaves as text/plain |
| Also carried in Mork | .msf, history.dat, formhistory.dat, panacea.dat |
| Last Thunderbird to use it | The 68 series, August 2019 |
| Converted by | Thunderbird 78, 17 July 2020 |
| Replaced by | abook.sqlite, with vCard inside it from Thunderbird 102 |
application/octet-stream, and the old version of this page said Binary outright.The magic line and the syntax come from the Mork grammar and primer, both written by David McCusker and still on Mozilla's archive server. The retirement date and the wording about conversion come from the Thunderbird 78.0 release notes.
The profile paths come from Mozilla's own support documentation. The file naming rule comes from Archive Team's file format wiki, which is the only reference that states it plainly.
Try a text editor. Free on all systems, and a MAB file is text. See section 4.
Look for the backup. Section 7 walks through the rename that restores it.
Using Thunderbird every day? Our Thunderbird error guides cover the faults that stop mail moving, from login failed to messages that will not delete.
Moving contacts somewhere else? Our vCard error guides cover what goes wrong on import, and ICS calendar files are the matching standard for appointments.
Moving the mail as well as the contacts? MBOX files are where Thunderbird keeps messages, and our MBOX error guides cover the faults that show up on the way out.
Coming from Outlook instead? PST files hold an entire Outlook mailbox, OST files hold the offline copy and OAB files are the offline address book that people mix up with this one.
Holding an older Mozilla or Mac mail file? OLK files come from Outlook for Mac and EDB files are the Exchange store behind a company mailbox.
Stuck on your computer rather than your file? Our Windows fixes and macOS guides cover the system side, including how to show hidden folders.
New to all of this? What a file format is covers the ground under it, and our file format guides list the rest.
Found a MAB file that does not match anything here? Send us its first line and we will tell you what wrote it.