Hi Chris,

The process is simple. It will take a little effort if you are new to Thunderbird. Conceptually:

1. Find your profile folder, then the local folders directory of it.
2. Copy your mbox file into that directory.

To find your profile folder:

1. Open Thunderbird.
2. Help menu in the application menu or click on the ≡ button at top right then "Help" > Troubleshooting information > Profile Folder > Show in [...]
3. Navigate to the "Mail" folder, then to the "Local Folders" folder within it. That's your destination.

Now:

1. Close Thunderbird.
2. Copy your mbox file to that "Local Folders" directory.
3. Open Thunberbird and look in the Local Folders account for that folder of imported messages.

Hi Rick! Thanks for the reply. It's still giving me the same error message. I even closed the app and re opened it and it still says the same message. Is there something I'm doing wrong? BTW thanks for the very clear instructions you made it very easy to follow along.

Oh, I left out one important instruction: remove the file extension (.mbox).

You don't want to open this file. Just open Thunderbird.

Did you copy the mbox file (and not move it)? Copying is better.

By the way, did you really want your allmail folder? You could have gotten more and smaller mbox files from Google Takeout and separate trash and spam files, so you would not have to import everything into Thunderbird. But your gmail account is no longer active, so you don't have that option now?