When my Thunderbird (78.10.1 - 64 bit / Windows 10) is in the online mode the CPU usage for the TB process gets very high and TB hangs (not responding). When starting in online mode it connects to email servers and download some emails but very quickly hangs. I have only few seconds after starting the program to change it to offline mode - then nothing bad happens and all functionalities (e.g. searching emails) work properly (similarly when starting TB in offline mode from the beginning). I tried restarting Windows, reinstalling Thunderbird (installing on existing version and in other step uninstalling and installing TB again), deleting msf files in profile folder (they where recreated but it didn't help), setting all accounts not to download emails from servers on start (doesn't help, after few seconds cpu goes crazy), running TB in safe mode and running Windows and TB in safe modes (with addons shut down), uninstalling Avast.