How to Open a Thunderbird MBOX File on Windows

Mozilla Thunderbird has stored email in the open mbox format for over twenty years, so your Thunderbird mail on Windows is already in a format any standard reader can open. Two things trip people up on a PC: the mailbox files have no extension, and they’re hidden away inside the AppData folder that Windows keeps out of sight. This guide shows you exactly where to look and how to read them with Mbox Viewer.

Mbox Viewer for Windows is available on the Microsoft Store — install it and follow the steps below.

Where Thunderbird hides your mail on Windows

Each mailbox (Inbox, Sent, your custom folders…) is a single mbox file inside your Thunderbird profile:

%APPDATA%\Thunderbird\Profiles\<random>.default-release\Mail\Local Folders\

The quickest way there: press Win + R, paste %APPDATA%\Thunderbird\Profiles and hit Enter — File Explorer opens right inside it. (%APPDATA% expands to C:\Users\<you>\AppData\Roaming, which is hidden by default, so the Win + R shortcut saves you from un-hiding folders.)

Inside Mail\Local Folders\ you’ll see files named Inbox, Sent, Archives and so on with no extension, each paired with a .msf file of the same name. The .msf files are just Thunderbird’s index — ignore them. The extension-less files are your actual mail, in standard mbox format. Mail tied to an IMAP account lives under an ImapMail\ folder instead.

Option 1: export with ImportExportTools NG (recommended)

The free ImportExportTools NG add-on is the tidiest route and works the same on Windows as anywhere:

1. In Thunderbird, install ImportExportTools NG from the Add-ons Manager.
2. Right-click any folder → ImportExportTools NG → Export folder.
3. Choose a destination — you get a proper .mbox file.
4. Open it with Mbox Viewer: double-click in File Explorer or drag it onto the window.

Option 2: open the mailbox file directly

No add-on needed — Mbox Viewer reads Thunderbird’s extension-less files as-is:

1. Close Thunderbird so the files aren’t locked.
2. Open the profile folder with the Win + R trick above and go into Mail\Local Folders\.
3. Copy the mailbox you want (e.g. Inbox) to somewhere easy like your Desktop. Always copy — never move the original, or Thunderbird loses track of it.
4. In Mbox Viewer, use File → Open and pick the copy. No need to add a .mbox extension.

What you can do once it’s open

You can search with the full query syntax, follow threaded conversations, preview attachments and export to EML, CSV or plain text. Mbox Viewer is strictly read-only, so your Thunderbird data is never altered. Even a 20-year-old, 30 GB inbox opens smoothly thanks to the streaming parser. New to the format? Start with how to open an MBOX file on Windows.