How to Import MBOX Files into Thunderbird 115 Supernova

Thunderbird 115 Supernova does not provide a universal “choose an MBOX file” command in its built-in Import wizard. For a one-time archive import, the most convenient route is the ImportExportTools NG extension version that explicitly supports Thunderbird 115. If you prefer not to install an extension, you can place the mailbox file in the active profile’s Local Folders store while Thunderbird is closed. Both methods preserve the imported messages as local mail; neither automatically uploads them to an IMAP server.

This guide is specifically for Thunderbird 115. The extension listing changes over time: the Thunderbird Add-ons site currently lists version 14.1.8 as compatible with Thunderbird 115.0–138.*, while its current version 15.0.1 is listed for Thunderbird 140.0–153.*. Do not assume the newest extension release is compatible with Thunderbird 115. Check the compatibility line on the official ImportExportTools NG version history before downloading. If this is a computer you use for regular email, consider moving to a currently supported Thunderbird release after importing; Thunderbird 115 is an older major version.

Quick choice: extension or manual copy?

Use Local Folders as the destination for an archive. This keeps old mail separate from the live Inbox and avoids accidentally copying imported messages to a mail server. First make a backup copy of the MBOX file, and, if the messages matter, back up the Thunderbird profile too. Mozilla explains how to find the active profile in its profile folder guide.

Before you import: identify the right file

MBOX is a mailbox container: it commonly holds many email messages in one file. It is not the same as an individual EML message, a Thunderbird address book, or a ZIP archive. An MBOX file may have no extension, or it may end in .mbox, depending on the program that created it. If you received a ZIP, extract it first and locate the actual mailbox file. Do not select a file ending in .msf; that is a Thunderbird index file, not the message archive.

• Keep an untouched copy of the original archive.
• If the source is a compressed ZIP, extract the contents to a folder you can find.
• Check that the file is not zero bytes and that you know which mailbox it represents, such as Inbox, Sent, or an archive folder.
• For a large archive, make sure the destination drive has enough free space for the expanded mail store and its indexes.

Method 1: import through ImportExportTools NG

1. Download a release that supports Thunderbird 115

Open the extension’s official version history and choose a release whose compatibility range includes Thunderbird 115. At the time this guide was checked, version 14.1.8 was listed for Thunderbird 115.0–138.*. Version 15.0.1 was listed for 140.0–153.*, so the newest listing is not the right choice for a 115 installation. Save the XPI add-on file to your computer. An XPI is the package file Thunderbird uses for an extension.

2. Install the XPI from Thunderbird’s Add-ons Manager

1. In Thunderbird, open the application menu and choose Add-ons and Themes.
2. Select Extensions in the Add-ons Manager.
3. Open the menu beside the extension search field and choose Install Add-on From File.
4. Browse to the saved XPI file, select it, and confirm the installation prompt.
5. Restart Thunderbird if it asks you to do so.

Mozilla’s add-on installation instructions describe the XPI installation flow. Install only an extension version from the official Thunderbird Add-ons listing. Add-ons can request broad access to mail data, so compatibility alone is not a reason to install a file from an unfamiliar download site.

3. Choose Local Folders as the destination

Return to the main mail window. In the folder pane, find Local Folders. If it is collapsed, expand it. Create a new folder there with a distinct name such as Imported Gmail Archive or Old Mail 2022. A clear destination name makes it easier to distinguish imported mail from messages in a connected account. Avoid importing directly into an active IMAP Inbox or a folder with the same name as a server folder.

4. Import the mailbox file

Right-click Local Folders or the destination folder you created, then open the ImportExportTools NG menu and choose its MBOX import option. The exact wording can vary by extension version. Choose the option for importing one or more MBOX files if you have standalone mailbox files. If your backup contains a directory tree with subfolders, choose the option that imports MBOX files together with their subdirectory structure, when available. Select the actual mailbox file or folder—not the ZIP file, not an .msf file—and confirm the import.

Import time depends on the archive size and the number of messages. Let the process finish before closing Thunderbird. A large archive can make Thunderbird appear busy while it builds folder indexes. Avoid starting a second import of the same file while the first one is still running.

5. Check the result before moving or deleting anything

Expand the destination under Local Folders. Open a few messages from different dates and verify that the sender, subject, date, body text, and attachments you need are present. Check message counts only as a rough comparison: mail exporters may include duplicates, drafts, or deleted messages, so counts can differ from the source application. Search for a distinctive subject or sender to confirm that older messages are searchable.

Method 2: copy one MBOX file into the profile

The manual method can work without an extension, but it edits files in Thunderbird’s profile. Do not use it while Thunderbird is running. A profile contains account settings and mail data, so first make a backup. Mozilla’s profile article shows how to open the active profile from Help → Troubleshooting Information → Profile Folder → Open Folder. Use that command rather than guessing which profile directory is active.

1. In Thunderbird 115, open Help → Troubleshooting Information.
2. Under Application Basics, locate Profile Folder and click Open Folder.
3. Close Thunderbird completely. Confirm it is no longer running before changing files.
4. In the profile folder, open Mail/Local Folders. If the folder is missing, create a folder in Local Folders in Thunderbird first, close Thunderbird, and then reopen the profile location.
5. Copy the MBOX file into Mail/Local Folders. Give it a unique name that does not collide with an existing mailbox. Thunderbird’s local mail store commonly uses extensionless mailbox filenames. If the source filename ends in .mbox, make a copy and remove that extension from the copy only, leaving the original archive unchanged.
6. Start Thunderbird and expand Local Folders. The copied mailbox should appear as a folder, and Thunderbird can build or refresh its index.

Do not overwrite an existing mailbox file, and do not delete or replace an existing .msf file to “make room.” If you cannot tell which file belongs to which folder, stop and use the extension method or create a new Thunderbird profile for the import. Manual placement is best for a standalone mailbox; preserving a hierarchy of nested folders may require matching Thunderbird’s .sbd directory layout and is easier to handle with the extension.

Common problems and what to check

• The add-on is marked incompatible: Check the Thunderbird version range on the extension’s version-history page. For TB 115, do not choose a release that begins at TB 128 or TB 140. Do not bypass a compatibility warning by modifying the XPI.
• The imported folder does not appear: Confirm you used the active profile, copied into Mail/Local Folders, and restarted Thunderbird. In the manual method, check whether the copied filename has an unwanted .mbox extension or a double extension hidden by the operating system.
• The folder appears empty: Verify that you selected the MBOX data file rather than an .msf index, a ZIP container, or a folder containing the archive. Keep the original and inspect the file size before trying again.
• Messages are present but attachments or formatting look wrong: The import transfers what is stored in the MBOX messages; it cannot restore attachments or content that were missing from the source export. Compare a few known messages against the original mail program.
• You expected mail to appear on another device: Local Folders are stored in this Thunderbird profile, not automatically synced to the email provider. If you need server-side copies, move or copy selected messages to an IMAP folder after verifying the local archive and considering the server’s storage limits.

Final verification checklist

• The original MBOX file remains unchanged in a safe location.
• The imported folder is under Local Folders, not mixed into the live Inbox.
• Messages from early and late dates open correctly, and expected attachments are accessible.
• A search for a known sender or subject returns expected results.
• The Thunderbird profile has a fresh backup before you reorganize or remove any source files.

For most Thunderbird 115 users, a compatible ImportExportTools NG release is the simpler way to bring in one or several MBOX files. Manual copying is a reasonable fallback for a single mailbox when you are comfortable working with profile files and keep Thunderbird closed during the change. Whichever method you use, verify the archive in Local Folders before deleting the source or moving messages into a live server account.