I AM WITHDRAWING THIS QUESTION, AS I THINK I DISCOVERED THAT THE PROBLEM WAS RELATED TO NEW MONITORING FEATURES I ACTIVATED IN MALWAREBYTES. I TURNED THEM OFF AND EVERYTHING IS WORKING FINE NOW. I believe there was an automatic update to Thunderbird a few days ago, and it looked like an error message popped up, but the message disappeared before I could read it. Ever since then, it's been taking forever to launch. Then when it finally does launch, I initially have a lot of lagging, delays, not responding messages, and even message windows closing while I'm in the middle of typing. It seems to "warm up" after a while and work as before. Any ideas what might be causing this and how to fix it? I'm on Windows 11. Thank you!