Is there any difference if you run TB in normal mode when OS X is running in safe mode? If not, do you have Spotlight enabled in Systems Integration or Global Search indexing ( both, in Preferences/Advanced/General), and if so, does it help to disable them?

Thanks for your reply sfhowes. Global search was enabled, I disabled it. No difference. I ran the Mac on safe mode, and even though the window was overly zoomed (impossible to see everything), it hung as in normal mode. I updated to Catalina, and that didn't fix anything.

Thanks many for looking into this and for working on Thunderbird; I really like it.

Cheers,

Juan

There is a similar discussion here, with a reference to a very long bugzilla thread. The issue is under investigation, but it appears there is no resolution at the moment.