problem solved but still cant understand how more people havnt been affected by this. may be due to staggered roll out around the world and some havnt got it yet or not installed it.
Or they don't encounter the problem at all.
As I said, I've installed the update on 3 computers and I use Firefox essentially on all 3. I have no problem maintaining login on my various sites, even those that use 2FA (gmail, hotmail, ....).
If you have the same problem with other browsers, it may be due to password manager (which I don't use) or something that went wrong with the update.
In this case, if in a while no-one has come to complain about the same problem, you might want to consider reinstalling avast properly.
Have you tried avast repair? It doesn't take long, so it might be worth a try.
https://support.avast.com/article/repair-antivirus/#pc