MAB is Thunderbird's old address book in the Mork format, the same odd text database behind the Netscape NAB covered elsewhere on this forum, abandoned by modern Thunderbird for SQLite. Two paths open it, one clean and one archaeological:

The clean path, an era matched Thunderbird as translator: install an older portable Thunderbird, versions before 78 read MAB natively, drop your abook.mab into its fresh profile folder over the empty abook.mab there, matching the filename exactly and open the Address Book window to find your contacts alive. From there Tools, Export writes CSV, the format Gmail imports directly. Portable builds keep this experiment out of any modern Thunderbird you actually use, and the whole translation takes minutes once the old build runs.

The archaeological path when installing anything old is unwelcome: MAB is text underneath, opening in an editor shows names and addresses scattered through Mork's markup, extractable by eye for a few dozen contacts into a spreadsheet saved as CSV. Mork parser scripts on GitHub industrialize this for larger books, the same tooling family the NAB thread describes.

Either path converges at contacts.google.com, Import, feeding the CSV, with the entries arriving under a dated Imported label for review and Google's Merge and fix reconciling any overlap with contacts Gmail already auto collected from your mail history over the years.