Have you checked to see if you still have Thunderbird installed?
It should be in C:\Program Files (x86)\Mozilla Thunderbird or the equivalent depending on the Windows version. If it was a portable install only you will know where that is or was.
How did the Thunderbird taskbar icon change? The Thunderbird icon hasn't not been updated in a very long time so unless you were using an old version, that is odd.
Do you update Thunderbird regularly and happen to know what version it is or was before it became no longer available? It should be v78.8.0 (32bit).
I'm wondering, only wondering, if when updating AVAST has installed the 64bit version. In which case you'll have it in C\Program Files\Mozilla Thunderbird and your existing desktop shortcuts will have had their icon changed to the 64bit version but could still be pointing at the previous C:\Programs File (x86) 32bit location.
You can change that yourself obviously but I'd point out that from Mozilla's own Thunderbird downloads page for the 64 bit version:-
"Please note that while the 32-bit and 64-bit versions of Windows can be used to run Thunderbird, only 32-bit builds of Thunderbird are supported at this time."
If the AVAST updater has done this, it should not have IMHO. Its the sort of reason why I hate automatic updating and updaters in general.
