Guys I have same problem, but for me Avast Passwords addon doesnt installing in firefox.
I tried on "clean" Firefox and on Firefox with addons.
Google is fine, but Mozilla isn't working.
Avast always asking to solve problem with Firefox with notice message. I do as recommended and nothing is happens. I also tried to re-setup Firefox and it's still not helped me.
I looked down for FAQ or youtube videos where people install addon for both Google and Mozilla and most of them is out of date. That faqs and videos is about earlier (old) version of avast and AVAST UI looks different.
My native language is Russian, i'm created topic here on ru-forum section but i'm think here will be faster.
Maybe you add install addon in the avast menu UI.
https://forum.avast.com/index.php?topic=207255.0 - link to the Ru-section, you can translate if you want with Google or etc.
Maybe moderators/Avast workers/Administrators will look by themselves on their own PC. I'm think you all have Avast and install Firefox is work for 5 mins.