How to Open a Thunderbird MBOX File on Mac

Mozilla Thunderbird has been storing email in the open mbox format for more than two decades. That’s great news: it means your Thunderbird mail is already in a format any standard mbox reader can open — including Mbox Viewer on your Mac. The only catch is that Thunderbird’s mailbox files have no file extension, so they don’t look openable at first glance.

This guide shows the two ways to get there.

Where Thunderbird keeps your mail

Each mailbox (Inbox, Sent, Archives, your custom folders…) lives as one mbox file inside your Thunderbird profile:

• macOS: ~/Library/Thunderbird/Profiles/<profile>/Mail/Local Folders/
• Windows: %APPDATA%\Thunderbird\Profiles\<profile>\Mail\Local Folders\
• Linux: ~/.thunderbird/<profile>/Mail/Local Folders/

Inside you’ll find files named INBOX, Sent, Archives and so on — without any extension — next to .msf files of the same name. The .msf files are just Thunderbird’s index; you can ignore them. The extension-less files are your actual mail, in standard mbox format.

Option 1: export with ImportExportTools NG (recommended)

The free ImportExportTools NG add-on is the cleanest route:

1. In Thunderbird, install ImportExportTools NG from the Add-ons manager.
2. Right-click any folder → ImportExportTools NG → Export folder.
3. Pick a destination. You get a proper .mbox file.
4. Open that file with Mbox Viewer — double-click or drag & drop.

This works for one folder or your whole account, and the export already carries the .mbox extension, so it opens directly.

Option 2: copy the raw mailbox file and rename it

No add-on needed:

1. Quit Thunderbird so the files aren’t in use.
2. In Finder, press Cmd+Shift+G and go to ~/Library/Thunderbird/Profiles/, then into your profile’s Mail/Local Folders/ directory.
3. Copy the mailbox file you want (e.g. INBOX) somewhere convenient, like your Desktop. Always copy — never move or rename the original, or Thunderbird will lose track of it.
4. Add the .mbox extension to the copy (INBOX → INBOX.mbox).
5. Open it with Mbox Viewer.

What you get in Mbox Viewer

Once open, you can search with full query syntax, browse conversations, preview attachments and export messages to EML, CSV or plain text. Mbox Viewer is strictly read-only: your Thunderbird files are never touched, so the copy is purely a precaution.

Files of any size work — a 20-year-old Inbox of 30 GB opens just as happily as a small archive, thanks to the streaming parser.