I suggest you look at the ImportExportTools addon. It will allow you to import mailbox files from another profile. It will usually be a two step process. If an account uses IMAP, you can't import directly into its folders. You can import into the Local Folders account and from there move the messages into the working account. I think I'd do the same with a POP-connected account too, so as to avoid creating new folders for the imported messages. https://addons.mozilla.org/en-GB/thunderbird/addon/importexporttools/