What Is Mozilla Thunderbird? Free Email Client, Versions and Where Mail Lives
Mozilla Thunderbird is the free email client from MZLA. Learn who owns it, which version is current, where it keeps your mail and how it compares with Outlook.
Mozilla Thunderbird is the free email client from MZLA. Learn who owns it, which version is current, where it keeps your mail and how it compares with Outlook.
Mozilla Thunderbird is a free desktop email client with calendar, contacts and chat. MZLA Technologies Corporation makes it as a Mozilla Foundation subsidiary.
Your mail sits in a profile folder as mbox files with .msf index files. The current Release is 159.0 from 27 October 2026 and the ESR line is 153.6.0esr.
Add IMAP accounts to move mail and copy the profile folder for local mail. Section 7 settles Mozilla Thunderbird against Outlook.
Mozilla Thunderbird is a free desktop email client. Calendars, contacts, chat, RSS feeds and newsgroups come built in. MZLA Technologies Corporation makes Mozilla Thunderbird and the code is open source.
You land here for one of three reasons. You want to know what the program is. Or you need to find your mail files. Some readers want to settle Mozilla Thunderbird against Outlook.
The Thunderbird email client reads and sends mail. It does not host a mailbox for you. Every version number below comes from thunderbird.net as of 7 Nov 2026. Where sources disagree this page names no number.
Mozilla Thunderbird is a free email client from MZLA. It keeps mail in a profile folder with one mbox file per mail folder by default. The current Release is 159.0. The ESR line is 153.6.0esr.
Weighing mail clients? Microsoft Outlook and Trojitá have their own pages in the email application guides.
MZLA Technologies Corporation makes Mozilla Thunderbird. It is a Mozilla Foundation subsidiary announced on 28 January 2020. An elected council oversees the budget.
The Thunderbird About page says the project began in 2003. It sits inside the Mozilla family. Mozilla Thunderbird now operates in a for profit subsidiary of the not for profit Mozilla group.
The January 2020 Thunderbird blog post names that subsidiary. On 28 January 2020 the Mozilla Foundation moved the project to MZLA Technologies Corporation. The post says the free and open source mission stays the same.
The Thunderbird Council acts like a board. The About page says contributors elected it and that it began in 2014. It oversees funds and approves budgets.
Thousands of individuals pay for the work through donations, per the same page. The Thunderbird email client for Android shows no ads. It earns no money from data, according to the Thunderbird mobile page.
The Wikipedia entry on Mozilla Thunderbird lists the chain. The Mozilla Foundation said on 26 July 2007 that an independent organization would develop Mozilla Thunderbird. Mozilla Messaging began operations on 19 February 2008.
Qualcomm and Mozilla announced a shared platform for future Eudora versions on 11 October 2006. Wikipedia lists that date. That link ties Mozilla Thunderbird to the old Eudora client.
Mozilla Messaging merged into Mozilla Labs on 4 April 2011. On 6 July 2012 Mozilla moved to Extended Support Releases. The community took over new feature work.
Mitchell Baker announced on 1 December 2015 that Mozilla Thunderbird would separate from Firefox. The Foundation stayed its legal home, per a 2017 post.
The K-9 Mail FAQ on the Thunderbird blog dates the K-9 Mail announcement to 13 June 2022. The team chose to work with an Android client instead of building a new one.
The Thunderbird 115 announcement of 11 July 2023 launched the Supernova design. It brought Card View, a unified toolbar and a redesigned calendar.
| Date | Fact | Source |
|---|---|---|
| 2003 | Project founded | Thunderbird About page |
| 7 Dec 2004 | Version 1.0 | Wikipedia |
| 19 Feb 2008 | Mozilla Messaging begins operations | Wikipedia |
| 28 Jan 2020 | MZLA Technologies Corporation announced | Thunderbird blog |
| 13 Jun 2022 | K-9 Mail joins Thunderbird | Thunderbird blog |
| 11 Jul 2023 | Thunderbird 115 Supernova | Thunderbird blog |
| 13 Nov 2025 | Version 145 with Exchange support | Release notes |
Mozilla Thunderbird handles mail, calendars, contacts and chat. OpenPGP and calendar arrived in version 78. Exchange support came in version 145.
Mozilla Thunderbird does more than send mail. Calendar, contacts, chat and encryption sit inside the app. Add-ons stretch it further.
Calendar and tasks became part of the app in version 78. The What's New in Thunderbird 78 post says so. Before that you installed the Lightning add-on.
OpenPGP encryption arrived in version 78.2.1. The OpenPGP page says the Thunderbird email client uses the RNP library. It no longer needs GnuPG by default. S/MIME works too.
Chat covers IRC and XMPP. Wikipedia adds Matrix since version 102. It adds CardDAV address books since version 91.
Exchange support came with version 145. The 145.0 release notes list Exchange through Exchange Web Services. Read Exchange Server to see what that server does.
Network features arrived in the same release. Version 145 added DNS over HTTPS. Its notes also list an Account Hub option for manual setup.
Version 115 brought a vertical Card View for the mail list. The older Table view stays available. A unified toolbar changes with the active tab.
The address book gained a tabular view and better keyboard support. You can also change density and font size from the app menu.
Version 91 added a built in PDF reader. It also added Apple silicon support, per Wikipedia. Version 13 added file linking to online storage. LDAP address completion works too.
The Thunderbird add-ons site lists extensions you install from the Add-ons Manager. Popular names there include ImportExportTools NG, Send Later, Quicktext and CardBook.
Steps follow the add-on installation page. Open the menu and select Add-ons. Search for the name and select Install. You can also drag an XPI file onto the Add-ons Manager tab.
Wikipedia says only MailExtensions work from version 78.7.1. Old add-ons written for earlier versions stopped working.
Account Hub is the account setup flow. Version 145 added manual configuration to it, per the release notes.
Version 159.0 fixed a case where Account Hub could not find CalDAV calendars for OAuth2 accounts. Section 7 sets Mozilla Thunderbird against Outlook.
| Feature | Fact | Source |
|---|---|---|
| Calendar and tasks | Built in since version 78 | Thunderbird blog |
| OpenPGP | Built in since 78.2.1 | Mozilla support |
| CardDAV address books | Since version 91 | Wikipedia |
| Matrix chat | Since version 102 | Wikipedia |
| Card View and new toolbar | Version 115 | Thunderbird blog |
| Exchange via EWS | Since version 145 | Release notes |
The Mozilla Thunderbird app is free forever. Thunderbird Pro is a separate set of paid cloud services that includes Thundermail.
Mozilla Thunderbird costs nothing. A donation is optional. The Thunderbird home page calls the app free forever. It asks you to donate if you can. Wikipedia lists the license as MPL 2.0.
Thunderbird Pro is a different thing from Mozilla Thunderbird. The Thundermail and Thunderbird Pro Services post of 4 April 2025 introduced paid cloud services. They sit apart from the app.
Those descriptions come from the 2025 post. The post says the Thunderbird email client is and always will be free.
The Thunderbird Pro April 2026 update puts most effort into Thundermail. It says Early Bird Beta invites would start going out the next month.
Prices changed and the post gives no figure. The team points readers to tb.pro for current pricing. This page quotes none.
Appointment gets reliability work and a simpler first time setup. Send gets visual and security improvements, per the same update.
The 2025 post says paid services will not be built into the apps on their own. Your mail client stays the same program.
Questions about a mailbox hosted elsewhere? GMX Mail and Yandex Mail cover two free providers that you can add as accounts.
Use Release for daily mail. The current number is 159.0. ESR 153.6.0esr suits you if you want new features once a year.
Four release channels exist. The Thunderbird download page shows Release at 159.0. It marks Release as the recommended choice for people who want new features first.
ESR means Extended Support Release. Both Mozilla pages say it suits you if you want new features once a year. Its add-on compatibility is the highest, per the release channel page.
Two Mozilla pages word the Release update rhythm differently. This page gives no number for it.
Beta lets you try upcoming features before they reach Release. It is more stable than Daily but bugs still appear. Bug reports from Beta users help development.
Daily holds the newest development builds. The release channel page warns that it can break. Data loss is possible. Contributors and testers use it.
The Mozilla version feed lists 159.0 as the latest version and 153.6.0esr for ESR. It lists 160.0b5 for the Beta channel and 161.0a1 for Daily.
The 159.0 release notes date the Release to 27 October 2026. The 153.6.0esr release notes give the same date. The 160.0beta release notes say beta 5 came out on 4 November 2026.
Wikipedia lists an older number as the latest stable build. Thunderbird.net is the primary source. This page follows thunderbird.net.
The notes list fixes for IMAP tags with non ASCII characters and for POP3 NTLM login. They also list security fixes for SMTP address line breaks and RSS headers.
Calendar and contact fixes appear too. A CardDAV sync stopped when a server rejected an old sync token. Version 159.0 fixes that.
Large ICS calendar files could use up memory. They could freeze Mozilla Thunderbird before 159.0. Yearly repeat events with several times in a day showed only the first time.
Forwarded mail could lose attachments. Printing could send many messages to the printer after you cancelled. Mozilla Thunderbird could also hang at startup. That happened when it restored IMAP and PDF tabs.
The beta notes list a Ctrl+A shortcut that selects all filters in the Message Filters dialog. They also say Office 365 setup no longer offers EWS automatically.
The system requirements for 159.0 list Windows 10 or newer. They list macOS 10.15 or newer. Linux needs glibc 2.17 or later and GTK 3.14 or later. The page lists 64 bit systems only.
Windows asks for 1 GB of RAM or 2 GB on 64 bit. It also asks for 500 MB of disk. macOS asks for 512 MB of RAM and 200 MB of disk. Mail needs extra space.
Windows 7 and 8.1 users get version 115.18.0 from the home page. The home page still shows a 32 bit Linux link. The requirements page lists 64 bit only. Check the download page first.
Distribution packages on Linux can carry other needs. The 115 post suggests the Thunderbird Flatpak from Flathub. It says update timing depends on your distribution. The home page also offers a Windows MSI installer.
Mozilla Thunderbird for Android sits on Google Play and F-Droid. Wikipedia dates the first stable release to 30 October 2024. That was version 8.0. The home page lists no iOS app.
The K-9 Mail FAQ said K-9 would stay maintained for about a year after Thunderbird for Android shipped. Old K-9 users get time to move.
The mobile page lists a unified inbox and light and dark themes. OpenPGP works through the free OpenKeychain app. You can move settings by scanning a QR code from the desktop app.
Mozilla Thunderbird supports both. IMAP is the default when your server offers it. You cannot change an IMAP account into POP later.
Mozilla Thunderbird selects IMAP when your server supports both protocols. The IMAP and POP FAQ says so. You choose POP3 in the incoming server list during manual setup.
Choose with care. The same page warns you cannot change that choice later. You add a new account instead.
IMAP keeps the mailbox on the server. A phone and a laptop can share one mailbox. The IMAP standard in RFC 3501 describes a client that works with messages on a server. Many devices can see the same folders.
The account options page says IMAP keeps only headers on your computer by default. Select "Keep messages for this account on this computer" and you get offline reading.
Under "When I delete a message" you can move mail to a folder. You can also mark it deleted. Or remove it at once. IMAP IDLE gives instant notice of new mail.
The option to clean up the Inbox on exit removes server mail that you marked as deleted. Mozilla Thunderbird does that when you close the app.
POP pulls mail down to your computer. The same options page lists "Leave messages on server". You choose a set number of days or keep them until you delete them.
"Fetch headers only" downloads headers first and the body when you open a message. "Automatically download new messages" follows your check interval.
The Message Store Type setting decides whether folders are single files or one file per message. You set it when you create the account.
Local Directory sets where Mozilla Thunderbird stores mail on disk. The Browse button lets you choose another location.
These steps come from the article on switching from POP to IMAP. Upload time grows with mailbox size. Keep old mail in Local Folders if your server has little space.
Choose Mozilla Thunderbird for free open storage and Linux. Choose classic Outlook for COM add-ins and full on premises Exchange.
Your account type and add-ins decide this choice. The table below lines up the checked facts so you can see where the programs differ.
| Question | Thunderbird | Outlook |
|---|---|---|
| Price | Free forever | Outlook for Windows has a free plan with ads |
| Local mail store | mbox by default, Maildir optional | PST and OST data files |
| Account types | IMAP, POP and Exchange via EWS | Work or school, Microsoft, Gmail, Yahoo, IMAP and POP |
| Exchange on premises | Added through EWS in version 145 | Classic full. New Outlook partial through IMAP |
| Add-ins | Add-ons site with MailExtensions | Classic supports COM and VBA. New does not |
| Platforms | Windows, macOS, Linux and Android | Windows for classic and new Outlook |
Prices and features come from the Thunderbird home page. They also come from the Outlook for Windows page. Account and add-in facts come from the Microsoft feature comparison.
Mozilla Thunderbird wins when you want free software on Linux. It also wins when open mbox files matter to you. Copying one profile folder also moves your entire setup between Windows and Mac.
Outlook wins when your company uses Exchange. It also wins when you need COM add-ins or VBA macros. Microsoft says classic Outlook supports them all. New Outlook does not.
Read Microsoft Outlook for the full Microsoft side. PST files and OST files explain its data files.
Microsoft rates offline support in classic Outlook as full. It rates new Outlook as partial. New Outlook imports mail, calendar events, contacts and tasks from a PST.
The PST page from Microsoft says new Outlook needs classic Outlook installed for PST work. It also lists a Microsoft 365 subscription as a requirement.
New Outlook has limits you should know. PST files do not work on the ARM version. New Outlook cannot start in Safe or Recovery mode. Multiple mail profiles are still listed as upcoming.
Web add-ins work in both versions for work, school and personal accounts. Neither version supports web add-ins for Gmail, Yahoo or iCloud accounts, per the same comparison.
Not by file. Mozilla Thunderbird does not open PST files by itself. The import page says Outlook imports need Outlook installed and a PST alone is not enough.
Add the same IMAP account to both programs. Both clients sync their own copies from the server. A shared account keeps the two in step.
Mozilla Thunderbird blocks remote content by default and supports OpenPGP. Keep it updated and review the telemetry setting.
Keeping Mozilla Thunderbird updated is your best defense. The 159.0 release notes list fixes for SMTP command injection and RSS header injection.
Those fixes show active security work. They also show why old versions carry risk. Update Release through the app or install ESR for steady yearly change.
The privacy panel page says the Thunderbird email client does not load remote content by default. Spammers use remote images to learn that your address works.
You can allow remote content for one message or for chosen sites. Allowing it for all mail is possible but gives up that protection.
The same page covers cookies. You can accept them from sites in mail you view. You can block third party cookies. A Do Not Track request is available too.
The scam detection page says the junk filter looks for phishing traits. It flags links with numeric addresses. It also flags links whose text differs from the real target.
Mozilla notes one limit. This detection does not learn from your email flags the way the spam filter does.
You choose encryption message by message. The OpenPGP page says Mozilla Thunderbird has no opportunistic encryption. It needs a public key for all recipients.
Keys sit in your profile directory. Set a Primary Password because without one those keys stay unprotected.
The Thunderbird privacy notice says Mozilla Thunderbird sends telemetry in two types. Interaction data covers how you use the app. Technical data covers your version and system.
Desktop and mobile users can opt out. The notice also says sending technical data briefly collects your IP address in server logs.
Does your company host mail on its own servers? Zimbra Mail and Zarafa have their own FileFix pages.
Add the same IMAP account to the new client and wait for the sync. Copy the profile folder or use the Import tool for local mail.
Moving mail depends on where the mail lives. IMAP mail waits on the server. Local mail sits in your profile folder.
Steps follow the article on moving Thunderbird data. Hidden files must show on Linux. Windows users copy the Thunderbird folder from AppData\Roaming.
Hold Option, open the Go menu and select Library. Copy the Thunderbird folder and share it by AirDrop or a drive.
Install Mozilla Thunderbird on the new Mac without opening it. Replace its Thunderbird folder with yours and test. Mozilla warns you not to reopen the old copy. Copy the folder back first.
Profile Manager lets you keep a profile in a custom location. On the new computer you create a profile. Then you point it to that same location, per the same page.
The import page covers imports from another Mozilla Thunderbird profile or an application like Outlook or from a file. Select Tools, then Import.
It carries accounts and messages and address books. It also carries calendars and settings. ZIP files above 2 GB fail.
Select "Import from another Thunderbird installation" for a big profile. Then choose a profile folder. Outlook imports need Outlook on the same computer.
Add your IMAP account to the new client. Then let it sync. For local mail read MBOX files. Mozilla Thunderbird stores folders that way. MAB address books explain older contact files.
Old clients have their own guides. Read Outlook Express and Windows Live Mail or Windows Mail. IncrediMail and Lotus Notes hold mail in other formats.
Small clients are covered too in SeaMonkey and Pegasus Mail. Mac users can open Apple Mail and the macOS guides.
Check folder size first and the .msf index second. Rebuild the search index last because it takes the longest.
Five faults cover most Mozilla Thunderbird support questions. Check them in this order.
Follow the Global Database page. Quit Mozilla Thunderbird and delete the file. Start it again and watch Activity Manager while it indexes.
Search returns a partial list until indexing finishes. Rebuilding does not shrink a large file unless you cut the indexed mail first.
The index covers mail and address book cards. It skips newsgroup posts. Mozilla Thunderbird stays usable while it builds a new index. It feels slower for a while.
Bugzilla bug 1279344 tracks reports of .msf files vanishing or going empty. Comments there say Mozilla Thunderbird rebuilds the file when you open the folder. A rebuild can reset columns and sort order.
Right click a folder and select Compact. The compacting folders page says this keeps all mail. Maildir folders need no compacting.
You can also compact an entire account from File, Compact Folders. Settings, General, Disk Space holds the threshold and the automatic option.
Mozilla suggests adding your Mozilla Thunderbird profile directory to your antivirus exclusions. Disabling automatic compacting is not recommended.
A profile still puzzling you? Send us the file details and we will check it with you.
Mozilla Thunderbird stores mail in a profile folder. Every mail folder is one mbox file by default with a .msf index beside it.
Mozilla Thunderbird keeps everything in a profile folder. The profiles page says that folder holds mail and passwords and preferences.
Windows keeps it under C:\Users\name\AppData\Roaming\Thunderbird\Profiles\. macOS uses ~/Library/Thunderbird/Profiles/. Linux uses ~/.thunderbird/. The profiles page also names ~/.mozilla/Thunderbird/ in its backup section.
The folder name carries a random eight character string. Default names include default-release and default-beta and default-daily, depending on the channel.
profiles.ini sits outside the profile and holds a Path entry for the profile. You edit that entry when you move a profile.
Mozilla Thunderbird uses mbox by default, which means one file per mail folder. The Maildir page says Maildir stores one file per message and stays experimental.
Maildir applies only to new accounts after you change the setting. Old accounts stay on mbox. Read MBOX files for the format itself.
The Maildir page calls the feature maildir lite. You cannot point an account at a server mail directory. Flags do not travel with the mail. Backups can shrink because only changed files need a copy.
An .msf file is the summary file for a mail folder. For POP and Local Folders it indexes the mbox file. For IMAP it holds message UIDs and cached header data, per Bugzilla bug 1279344.
Bugzilla bug 1576525 moved new profiles from abook.mab and history.mab to abook.sqlite and history.sqlite. It targeted Thunderbird 71. Older profiles keep the .mab pair.
profiles.iniprefs.js.msf index beside itglobal-messages-db.sqliteabook.sqliteabook.mabopenpgp.sqlite| Item | Role | Source |
|---|---|---|
| Profile folder | Holds your data | Mozilla support |
| .msf file | Folder summary and index | Bugzilla 1279344 |
| Global Database | Search index | Mozilla support |
| abook.sqlite | Default address book | Bugzilla 1576525 |
| openpgp.sqlite | Key acceptance | Mozilla support |
For background on formats, read what a file format is. The file format guides list all formats written up so far.
Check the account type first. Section 9 shows both routes.
Check the index files. Section 10 lists the fixes.
Choosing between Mozilla Thunderbird and Microsoft's client? Microsoft Outlook covers the Microsoft side. PST files and OST files describe where Outlook keeps mail.
Holding old mailbox files? MBOX files explain the format Mozilla Thunderbird writes. MAB address books explain contact files in older profiles.
Managing a company mail server? Exchange Server covers Microsoft's server. EDB files hold Exchange data.
Coming from an older client? Outlook Express and Windows Mail have guides. Windows Live Mail sits in the same list.
Wondering about other mail programs? Sylpheed and Opera Mail have pages. Spicebird and Protonmail do too.
Looking for something else? Browse the email application guides or the Windows error guides. Send us the file details when Mozilla Thunderbird behaves in a way this page leaves unanswered.