Been using Thunderbird (now 38.5) for as long as I have had a MS based computer(I think but cannot remember when) Recent update to Win 10 from win 7 set Thunderbird to "not responding" . Eventually it clears but requires some patience. Once it clears the first time, Thunderbird seems to be OK. Here is the solution that worked for me. Test Avast after fresh reboot before opening Thunderbird Open the lower right carrot to view hidden icons. Close all unnecessary programs like io bits etc. Right click Avast icon. Avast shields control>disable 10 minutes. Open Thunderbird. If it does not hang or respond slowly. Avast v Thunderbird is the problem. Set an exclusion in Avast worked for me. Open hidden icons Right click avast Click settings gear-general-scroll down and open exclusions Insert the path and folder/file name for your Thunderbird profiles. C:\Users\Windows user name\AppData\Roaming\Thunderbird\Profiles\*.* worked for me. I have two profiles: one for my wife and and another for me so I used the *.* wildcards I think the general page of Thunderbird and AV programs wrote that the scanning of the profiles is not necessary since emails are scanned on arrival and there is no need to scan the profiles. Regardless I hope thye problem is fixed without the exclusion. Good Luck.