In Thunderbird this usually means the folder's index file (the .msf) is damaged, or the MBOX folder hit a size limit. The index is rebuilt from the mail, so fixing it does not lose messages. Jump to your situation below or work through the methods in order.
In Thunderbird, 0x80004005 (NS_ERROR_FAILURE) on an MBOX folder almost always means the folder's index file is damaged. Every MBOX folder has a companion .msf file that indexes the messages for fast display; the actual mail lives in the MBOX file itself. When the .msf is corrupt, often after an unclean shutdown, an antivirus lock or a disk hiccup, Thunderbird cannot read the folder and raises this generic failure. Because the .msf is only an index, the fix is safe: use Repair Folder (or delete the .msf with Thunderbird closed) and Thunderbird rebuilds it from the MBOX with no loss of mail. Compacting the folder and checking the disk clears the remaining causes. An oversize folder can also trigger it, in which case splitting the folder helps.
Thunderbird stores each folder as a plain MBOX file plus a small .msf index that lists where each message sits. The index makes the folder open quickly, but it can fall out of sync or corrupt. When it does, Thunderbird cannot map the folder and reports the generic NS_ERROR_FAILURE, 0x80004005.
The reassuring part is that your mail is in the MBOX file, not the index, so rebuilding the index is safe. Repair Folder deletes and regenerates the .msf from the MBOX. The same error can come from a folder that has grown very large or from disk errors, which is why splitting a huge folder and checking the disk are the follow-up steps.
“This scares people because the folder just will not open, but the mail is almost always fine. Thunderbird keeps the messages in the MBOX file and a separate .msf index, and it is the index that rots, usually after a crash or an antivirus grabbing the file. Right-click the folder, Properties, Repair Folder, and Thunderbird throws the bad index away and rebuilds it from the actual mail. Nine times out of ten the folder opens straight afterwards with everything intact.”
.msf file (same name as the folder, with a .msf extension) and delete it, leaving the extension-less MBOX file in place.Your mail is in the MBOX file, not the .msf index, so Repair Folder is safe and fixes the large majority of cases. If it recurs, the usual cause is antivirus locking the profile, so exclude the Thunderbird profile folder from scanning. Compacting regularly keeps the index healthy and prevents the error returning.
If Repair Folder and a fresh .msf do not help, the MBOX file itself may be damaged rather than the index. Make a copy of the MBOX file first, then re-import it through ImportExportTools NG, which can often read past a damaged section and recover the messages. You can also submit your error to us for a tailored fix.