Avast just posted they had dropped support for Firefox

What did the Avast add-on do? Many of the major security software vendors have browser add-ons that add a little extra to their core features/functions, which run outside the browser. It's not clear which of those add-on extras are important to you.

See:

• https://support.avast.com/en-us/article/aosp-firefox-discontinuation-faq/