This article works with the version, 14.1.17 (64-bit).
- Visit Add-ons and Themes. Click +Download Now to download and save Version 14.1.17 for ImportExportTools NG.
- From Thunderbird's menu bar, select Tools and click Add-ons and Themes. If your menu bar is missing:
- Windows: Right click at the top to click Menu Bar and Mail Toolbar.
- Mac: Move your cursor all the way to the top of the screen; Click the green circle to resize the window.
- Click once on the Gear to select Install Add-on from File.

- Locate and click to install ImportExportTools NG. Click Add to add the extension.
- Enter ImportExport in the box, Find more add-ons, press Enter (Windows) or Return (Mac) on the keyboard.
- Restart Thunderbird once the add-on is installed.
- Visit Import a MBOX File into Gmail with Thunderbird to continue with importing a mbox file.