See https://support.mozilla.org/en-US/questions/1285736

If you got the mbox file from Google Takeout, it will probably have an .mbox file extension. First, remove the .mbox extension, so the file is of Type File in File Explorer. Then, open the profile folder from Help/More Troubleshooting, Profile Folder, Open Folder, close TB, and copy the renamed mbox file into the Mail/Local Folders location. Restart TB and find the folder under Local Folders in the Folder Pane.