have you actually tried to interact with Thunderbird when this is going on?

Absolutely, that's the problem. I can't interact with Thunderbird at all, the colored spinning wheel won't allow me to even quit out of Thunderbird. The only way to make the spinning wheel stop is force a shut down of the entire computer. It's only spinning in Thunderbird, I can use the rest of the computer (but I haven't used the entire computer for long, I usually shut everything down shortly after when the wheel starts).

Thanks for responding.

am not mac person, but that sounds like it might be a disk error, especially if the operating system goes down shortly after.

Try the instructions here http://www.dreevoo.com/content.php?id=1402 See if the disk or file contain allocation or physical errors.

No, the rest of the computer works fine. This only happens to Thunderbird, in Thunderbird. The only reason I've mentioned the rest of the computer is because the only way to make the wheel stop spinning in Thunderbird is to force the entire computer off. I can't turn it off normally, as Thunderbird is "running" and won't stop.

Have you checked the file system. your response does not remove the need to do so.

I'm sorry, what file system are you referring to?

Macs aren't like PCs (as you've already pointed out), I believe I would need to use a program called Terminal to get into the file system, and I only really know how to do that to create epubs… unless I'm misunderstanding you?

I gave you a link to what you needed to do back here https://support.mozilla.org/en-US/questions/1133144#answer-902563

What add-ons / tweaks are / were enabled in tbird? Have you also considered the idea it's not tbird that derped but incompatibility with an addon + latest update.. One simple way to check is launch tbird in 'safe mode' directions for how to achieve this outlined here: https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

Matt, I apologize for not understanding your last post. Yes, I run firstaid when I backup twice a month, and ran it again just now. That says everything is fine.

Corey, I don't have any addons (if you mean to Thunderbird). I know this may produce a groan, but I hate allowing updates because everything changes… so I put off the last update until this wheel thing started happening so frequently, and I did update. That update didn't help at all, if anything, made it worse.

What I do have is a subscription to Creative Cloud, which I'm wondering might be the cause—Adobe has been trying to force me to use Chrome and Safari. I will never put Chrome on my Mac, I have Safari but only use it because Adobe forces me to for CC. Do you think that maybe Adobe has done something to make Mozilla products incompatible? The last time I called tech support, the support person wouldn't allow me to use Firefox at all (was making me use Safari) to the point where I hung up and called back to get someone who was more Firefox friendly, and I did get someone who was Firefox friendly…