Is it blank in safe/troubleshoot mode (hold Shift when launching TB)?

Hmmmm...no, in Troubleshoot mode, it works just fine. So what does that imply??

It means there is either an incompatible extension or theme, or that hardware acceleration should be disabled (Preferences/General/Indexing) or the layout is broken (launch in safe mode, 'Reset toolbars & controls', 'Make changes & restart').

Disable acceleration in normal mode, restart, and see if it works properly. If not, do the reset. If it still doesn't work, look for an incompatible add-on.

OK...disabling acceleration didn't do anything, but when I launched in Safe mode and reset the toolbars, that seemed to take care of the problem...BUT after testing various things, I discovered that when I use the "Identity Chooser" extension, I get the blank original message. When I disable Identity Chooser, the original message is there. Why would the recent update make this extension wonky?? Identity Chooser was an extension I really liked and used all the time!

Do you have the latest version, 3.1?

https://addons.thunderbird.net/en-us/thunderbird/addon/identity-chooser/reviews/1167658/

That solved the problem!!! I guess I was under the impression that extensions automatically updated like T'bird...from now on, I'll be careful to check for updates to the ones I use!

Thanks SO much for your help!