Where Thunderbird stores mail on a Mac

Thunderbird keeps everything in one profile folder at ~/Library/Thunderbird/Profiles/, inside a directory whose name starts with eight random characters. Mail, settings, address books and accounts all live there together, which makes the profile the only thing worth copying.

That single folder is why Thunderbird moves between Macs more easily than any other client. It is also why a half copied profile fails so badly.

The path

Press Shift and Command and G in Finder, then enter this.

Inside is at least one folder named something like a1b2c3d4.default-release. The eight characters before the dot are random, generated when the profile was created, so yours will not match anybody else's.

Thunderbird will point you at it directly if you would rather not type. Help, then Troubleshooting Information, then Open Folder next to Profile Folder. That opens the profile itself, so go up one level for Profiles and one more for the Thunderbird folder.

Library is hidden by default. Typing the path reaches it anyway. The notation itself has its own guide. The folder does not appear in Finder until you unhide it, which is true of everything else in there too.

profiles.ini, the file that decides which one loads

One level up from Profiles sits profiles.ini, a plain text file listing every profile Thunderbird knows about.

Each entry carries a Path= line and an IsRelative= line. Relative means the path is read from the Thunderbird folder. Set it to zero and the path is absolute, which is how a profile gets stored somewhere else entirely.

This matters when a profile moves. Copying the folder is only half the job. If Thunderbird still has the old location written in profiles.ini, it starts up and finds nothing, which reads exactly like lost mail and is not.

Mail and ImapMail, the two that hold messages

Open the profile and two folders hold messages.

The split matters when you are deciding what is worth rescuing. Everything under ImapMail exists on a server as well. Everything under Mail, Local Folders most of all, exists nowhere else.

Get that split wrong and a Thunderbird migration loses the only irreplaceable half. The same trap sits under every client switch rather than this one.

The files with no extension

Open Mail/Local Folders and the contents look broken. Files called Inbox, Sent and Archives with no extension at all, sitting beside files of the same name ending in .msf.

Nothing is broken. The extensionless files are the mail.

The .sbd arrangement has a trap in it. A folder that only contains other folders still needs its own mbox file, empty though it is. Without it Thunderbird cannot see what is inside the matching .sbd directory. Copy the .sbd folder without its namesake and the subfolders vanish from the interface while sitting on disk.

One more case looks alarming and is not. An IMAP account set to keep no offline copies has an .msf and no mbox at all, because nothing was ever downloaded.

What those extensionless files actually contain, plus why applications disagree about reading them, belongs to the MBOX format itself.

What else the profile is holding

Mail is the part people come looking for. It is not the part that takes longest to rebuild.

Rebuilding a set of filters and re entering half a dozen account passwords takes an afternoon. Copying the folder that already contains them takes a minute. That asymmetry is the entire argument for treating the profile as one unit.

Moving a profile to another Mac

Take the profile folder entire. Not the Mail folder, not a handful of mailboxes, the whole directory.

Everything is in there together. Accounts, server settings, filters, signatures, address books, saved searches, add ons and the mail. Take a subset and you rebuild the rest by hand.

Quit Thunderbird first. Copying a live profile catches files mid write. The result imports with folders that will not open.

On the new Mac, put the folder inside ~/Library/Thunderbird/Profiles/ and let Thunderbird find it. Failing that, point profiles.ini at wherever you put it.

Mozilla is explicit about one thing. Simultaneous access to any part of a profile folder, through a file share, cloud storage or anything similar, is not supported and risks corruption and permanent loss of data. A profile that lives in a synced folder and gets opened on two Macs at once is the documented way to destroy it.

What to copy for a backup

The same answer. The profile folder, with Thunderbird closed.

It compresses well. It restores by being put back. It carries no dependency on the version of Thunderbird that wrote it beyond ordinary forward compatibility.

What it is not is an archive. The profile is readable by Thunderbird and by very little else, so a copy kept for years is a copy that needs Thunderbird to still exist and still run on whatever Mac you own then. Mail you want readable in a decade wants to leave the profile in a format other things can open.

The short version

One path to remember, ~/Library/Thunderbird/Profiles/. One file that controls it, profiles.ini. Two folders that hold mail, of which only one holds anything the server does not have. Files without extensions are the messages and files ending in .msf are disposable.

Copy the profile whole with the application closed. Almost nothing can go wrong after that.