I would test to see if any addons are causing a conflict by starting Thunderbird in safe Mode. In thunderbird Help > restart with addons disabled

then test to see if the 'black' box has disappeared. If yes, then one of your addons is the cause. You would need to disable one addon at a time to locate the problem addon. It might need to be updated.

That's part of the problem; the drop-down box for "help" is black (see attached jpg). Any other way to start in safe mode?

xref https://support.mozilla.org/en-US/questions/1043370