---
title: "How to Open a Thunderbird MBOX File on Mac — Mbox Viewer"
description: "Thunderbird stores your mail in standard mbox files — they just have no extension. Here are two easy ways to read them on macOS with Mbox Viewer, without importing anything."
url: https://mboxviewerpro.com/guides/how-to-open-thunderbird-mbox-on-mac/
language: en
updated: 2026-06-11
source: Mbox Viewer
---

[All guides](https://mboxviewerpro.com/guides/)

# How to Open a Thunderbird MBOX File on Mac

Thunderbird stores your mail in standard mbox files — they just have no extension. Here are two easy ways to read them on macOS with Mbox Viewer, without importing anything.

PublishedJune 11, 2026

Mozilla Thunderbird has been storing email in the open [**mbox**](https://mboxviewerpro.com/glossary/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 — but Mbox Viewer opens them directly anyway.

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](https://addons.thunderbird.net/thunderbird/addon/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: open the mailbox file directly

No add-on needed — Mbox Viewer opens Thunderbird’s extension-less mailbox files as-is, no renaming required:

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 the original, or Thunderbird will lose track of it.
4.  Open the copy with **Mbox Viewer** — double-click or drag & drop. No need to add a `.mbox` extension.

## What you get in Mbox Viewer

Once open, you can [search with full query syntax](https://mboxviewerpro.com/guides/search-syntax-cheat-sheet/), browse conversations, preview [attachments](https://mboxviewerpro.com/glossary/attachment/) and export messages to [EML](https://mboxviewerpro.com/glossary/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](https://mboxviewerpro.com/glossary/streaming-parser/).

## Open your archive with Mbox Viewer

Native Mac and Windows app. Streams MBOX and EML files of any size, fully offline.

[Mac App Store](https://apps.apple.com/app/mbox-viewer-pro/id6759237715) [Microsoft Store](https://apps.microsoft.com/store/detail/9NW3GVFG7DDB)