I have this problem too Thunderbird is now dying multiple times a day - When I send an email I get the spinning wheel of death, then have to force quit. And I agree it seems to have come up when I moved to 10.14 or 10.14.5. Can anyone figure out what may have happened? I looked through the support posts and nothing seems to work.

This is happening for me as well. I've been on Mojave for a while without any problems. It started happening for me after macOS 10.14.6 recent supplemental update or Thunderbird 60.8.0, just a few days ago. Randomly happens after sending a email. Doesn't happen after every sent email.

I had read somewhere that any folder sizes over 2 GB may contribute to this. I archive all of my read mail into a local "Read" folder and remove it from the server, and I have rules set up to move mail older than 6 months into a separate "6-12 months old" folder, and anything older than 1 year gets purged. However, my "Read" mail that is less than 6 months old is around 3.1 GB, after compacting just now (as a print/web designer, I'm constantly sending/receiving large attachments).

Needless to say, I have the same issue as you all -- I'll send an email, then go to grab my main TB window (it's typically minimized to the dock), and it's then that I notice the spinning beach ball of death.

For now I just live with it and do as you all are doing -- force quit and restart.

Would love a better solution, since I can't quite get all of my folders down below the 2 GB threshold at the moment. *shrug*

There may be a bug report on this, but can't remember.