Is it only happening when you send email?

Thanks so much for responding. No, it can happen any time. For example, I can be away from my computer, come back, and it has hung. But it often happens while reading and sending email, in particular just after I've sent an email. BTW About Thunderbird says that it's up to date, on release update 68.10.0.

If it is not https://bugzilla.mozilla.org/show_bug.cgi?id=1381485#c349 then you might consider possible hardware issue.

If you are not using Enigmail or other addons, and not queasy about using beta, you might try it.

Wayne -- I'm doubting that it's a hardware problem, because it occurs on both my iMac workstation and a laptop at home. And it also happened to someone in technical support in my office. See his email below. Understanding the link you sent me was a little beyond my expertise. By any chance do you have an explanation of what I can do in plain English? Or will this issue be fixed in a future release? Susan

"It is an early 2008 Mac Pro using Mac OSX El Capitan. It was mostly switching between other applications or the desktop to the main TB window. It may have occurred at other times but this is the only one I can really recall."

Rod Prieto Digital Media Specialist Paul G. Allen School of Computer Science & Engineering University of Washington

Wayne -- I though it might be helpful if I kept better track of exactly when Thunderbird hangs, so you can narrow down what the issue is. I can do this today and over the weekend and get back to you on Monday, if that's ok with you. Susan

a Duplicate thread started here https://support.mozilla.org/en-US/questions/1294975

Wayne -- This was my original message: Thunderbird hangs several times a day. I have to ForceQuit it and restart. I've sent a bizillion emails to Apple, but no response or fix from them. This happens on all my Apple devices, both laptops and a workstation. Can you help? Thanks very much. Wayne -- Thunderbird logged me out over the weekend so I have to start a new request. I was eggers26 for our last conversation. I have more specific information now about when TB hangs. 7/9 iMac: Hangs right after my reply to an email; my reply made it to the Sent folder. 7/10 laptop: same thing, 3 times. 7/11 laptop: same thing, 2 times. Do we suspect the sender?  :-) I also have a dump on the laptop that I could send you. The stack, thread activity, binary images, all in hex. Various id info and timing in English. Thanks, Susan