I have been suffering with "Not Responding" messages when using Thunderbird 91.3.2 (64bit) on a Dell Optiplex 9010, 16gb ram, 2 gb sata hard drive, Win10 Pro, V 20H2, for many months. It has been so terrible I have started to search for different email programs. I use FireFox V 18.5. Searching many forums and many web assistance sites I have been advised to do many-many things -- probably 8 to 10 different things advised to try and I did ALL OF THE SUGGESTIONS -- NOTHING helped ever!! Last thing to be tried, recommended by Thunderbird forum, was to start killing different programs that operate in my system and restart each time and test Thunderbird then "on each one" separately. That would be a long process you can bet. Today I was seeing some difficulty with FireFox -- very slow and doing some things not normally seen. I decided I would just COMPLETELY remove FireFox and Mozilla Maintenance Service from my system. Once done, I thought I would try the old Thunderbird Email again and see if it was still having its "Not Responding" headaches. Well, low and behold-- Thunderbird is working perfectly. Primarily the problem was it would not allow me to do "Any Actions" on the email program itself or any other action on my computer while the Thunderbird was downloading new email messages. But with FIREFOX UNINSTALLED (and now defaulting to MS Edge) Thunderbird is sailing along -- downloading messages excellent -- and I can do other things while waiting on my computer without it locking up. So -- the bottom line seems like this. All my time trying to find out what rascal of a program in my system was attacking Thunderbird was a HUGE waste of time. All the time it was Mozilla's own FireFox causing some conflict with its own Mozilla Thunderbird. All users have to do is "Uninstall FireFox" and their Thunderbird "Not Responding" headaches will end totally. At no time in weeks and hours of research on the internet did I ever see anyone comment that FireFox itself might be causing the problem with HANGS. I will try to reinstall FireFox again as I have always depended upon it as being the safest browser I could choose but if Thunderbird "Not Responding" ever pops up again I will have to switch to MS Edge or other. I know Thunderbird has been undergoing a lot of difficulty as it is held back by a lack of Donations being made. I have donated myself a half dozen times over years. I would highly recommend if the Thunderbird management team can get to read this email that they look very hard at why FireFox would be conflicting with Thunderbird as nobody is even speculating that this might be the source of the "Not Responding" problem and from what I see most everyone with the problem on various internet sites and forums "IS NOT" getting the issue resolved. Who would think that FireFox itself is the basis for the Thunderbird trouble and not have been explored by the Thunderbird management team!! I guess my Thunderbird problem is solved but FireFox has some issues to keep an eye upon I would say. GPTDesign47 OS Version: Microsoft Windows 10 Pro, 64 bit, Build 19042, Installed 20210204072927.000000-300 Processor: Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz, Intel64 Family 6 Model 58 Stepping 9, CPU Count: 8 Total Physical RAM: 16 GB Graphics Card: Intel(R) HD Graphics 4000 Hard Drives: C: 1861 GB (1680 GB Free); F: 149 GB (147 GB Free); H: 465 GB (224 GB Free); Motherboard: Dell Inc. 0M9KCM, ver A00, s/n /8S3HWV1/CN7220027F011Q/ System: Dell Inc., ver DELL - 1072009, s/n 8S3HWV1 Antivirus: Windows Defender, Disabled, Norton 360 Premium 1 installed, Malarebytes Pro installed