does holding the shift key while starting to enter safe mode change that?

Yes, I was able to open the folders and access the menu. I closed Thunderbird and opened it again and all seems well. What would cause it to act up like that, any idea?

It is a know fact that on occasions starting in safe mode clears issues all by itself. However as no one ever identifies the actual original issue then what is cleared never gets found. After all starting in safe mode is the first troubleshooting step to disable rogue add-ons.

I am guessing when I say it is probably to do with updating the links to add-ons or actually updating add-ons. For example, Lightning updates and Thunderbird will try and use the old version until Thunderbird restarts. It is only a small step to that update needing to have one or another add-on disabled to complete.

Well, whatever it was it is gone, and I'm grateful for your assistance! Now I know what to try first if the issue returns. Thank you!