Start Windows in safe mode with networking enabled - https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still In Windows safe mode, start Thunderbird in Troubleshoot Mode - https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

Does problem go away?

Hi Wayne, thanks for the help. Starting in Troubleshoot mode while in Windows safe mode improved the problem, however I am still experiencing sporadic and long freezes when I do actions in the app such as open mail.

I also experienced extreme performance degradation after upgrading to 128. Deleting a mail or feed item in a folder with 3-10 items freezes Thunderbird for 2 seconds. Deleting a feed item in an RSS folder with 900 items freezes Thunderbird for 30 seconds. Similar size-dependent delay when opening folders. Thunderbird troubleshoot mode seems to reduce the delay by 0-70% (which still means >10s delay for big folders!), but I'm getting very high variance so can't say for sure before more investigation.

@gallium: By chance, does your Thunderbird open a very big mail folder immediately when starting?

see https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

@gallium: By chance, does your Thunderbird open a very big mail folder immediately when starting?

The folder Thunderbird opens into has about 3000 messages, which seems not unreasonable to me. Other mail apps I use open into folders of 4000 messages.

see https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

I will try these options if the app becomes unusable again, thank you.

It turned out that my issue was 57k mailnews.tags.* entries in prefs.js. I had the option "Automatically create tags from feed category names" turned on in my news feed and it had accumulated a lot of tags over time, saving a config preference for each. After editing prefs.js directly and removing all mailnews.tags.* lines, there are no freezes when opening folders or deleting items.

And to clarify, this worked fine before version 128, so I'd consider it a regression (though some may consider it https://xkcd.com/1172/), but one easily worked around. :)

Thanks for the update.

Are you using Cards View?

@Wayne: Yes, I am and was using Cards View. I tested that the freezes occurred with and without the Message Pane, but didn't test if Table View had any effect.