Follow such steps to import Gmail Archive MBOX file to Thunderbird. First, install Thunderbird on your Windows Computer and run it.

The default setting for Windows is to not display a file's extension. Therefore, when viewing files in Windows you would only see the portion of the filename that precedes the last period in ...

Open the app, click on File, and click on “Import Mailboxes.” Make sure “Files in mbox format” is selected, and then find your .MBOX file (s).