It's hard to pin down when exactly it will happen but Thunderbird regularly freezes from roughly 10~15 seconds after clicking the red close button. In that time TB doesn't allow the desktop or any other app to become active until it finishes whatever it's doing. The bug occurs after I've brought it to the front and then close it. For example, an email just arrived so I checked that then closed TB. That's when it hung. I've found it also happens after discarding a draft so that I'm unable to open or even highlight other emails in my Inbox for a short time. This hanging started happening right after I upgraded to Mac OS X 26.2 a few weeks ago. So far I've tried... 1) Troubleshoot Mode. No change. Even with no extensions or themes installed the problem shows up. 2) Increased the value of mail.db.idle_limit 10X to 3000000. 3) Did a clean reinstall of the app (v146.0.1). I redid Settings from scratch and imported emails from a previous installation. The size of the Profiles folder is 991MB. 4) no anti-virus software is installed. It's similar to this bug. I made a comment there 2-3 weeks ago but no reply so far. https://bugzilla.mozilla.org/show_bug.cgi?id=1998881 Also similar to this... But I don't have a calendar installed. https://bugzilla.mozilla.org/show_bug.cgi?id=2000460 Not sure what else to try. Any suggestions?