I have the same problem, but have had it a lot longer than just the most recent update. Thunderbird for me (on my Mac) is a hungry beast, it regularly consumes 94% of CPU resources and balloons up to 2GB of memory. It means that every few minutes, Thunderbird will lock up and have the spinning ball of death for a minute or so... the Activity Monitor during that time says the app isn't responding and its consuming 94%+ CPU and rushes up to 2GB of memory within a few minutes of re-opening the app. It makes my otherwise super fast laptop slow. I've tried everything... I now only download to offline messages in the last year, I updated the idle time to 30000000 rather than 300000, and every other fix I could find online. Any advice? Thanks!

better in safe mode ? https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

Since it doesn't happen every time, I can't use safe mode to test. When it does happen, Thunderbird is frozen and I can't test safe mode. Thankfully when I do force quit and restart, it usually does fine, though if I'm been working on an email, it may be lost. Interesting to hear Leithyn's experience with it consuming memory. Mine has done that too, but I thought it was because I had too many emails. I emptied the trash, but that was before the update and the freeze problem.

I forgot to say thanks for responding.