I have been using Thunderbird and Firefox for many years and have never had any Speed issues until now. I installed macOS Big Sur (v10.0.1) and upgraded tp Thunderbird (v78.4.3) (64 bit) this past week and immediately noticed that Thunderbird was "Sluggish". For instance, when I take my mouse pointer to an email(s) that I want to delete, it takes 5-10 seconds for that email(s) line to be "Highlighted" and once it does, it takes another 3-4 seconds before I can see the "Delete" button or can use my Keyboard's "Delete" key. I have absolutely ZERO problems with any other Apps (including Firefox) on both the Mac side and Parallels Desktop for Mac. I am hoping that the programmers are working to fix this annoying problem.