I’ll spare you the Thuderbird installation and first-time setup instructions
1. Download, launch Thunderbird
2. Mail Account Setup
3. Find your Thunderbird “Local Folders” Directory
Click on “Local Folders” and then “View Settings for this account” to see where it is looking for local mail folders. Something like this:
(You may have to allow “Show hidden files, folders, and drives”)
on your Mac
4. Drop the .mbox file into Local Folders
Quit Thunderbird. Navigate to your “Local Folders” directory. Drag and drop your mbox file in there. You may change the name to something appropriate (something like mbox-harsha-20161009-gmail).
5. Browse mbox
Restart thunderbird, and you should see the mailbox in your Local Folders list.
Feel free to leave comments.