Was somebody able to install Avast Online Security addon?
It does not show up in 64bit ver.
Go to my.avast.com and wait a second for popup to appear at the bottom of the page. Use that to install addon.
Awesome! 
Finally I can enjoy 64bit web browsing on Firefox.
You can enjoy web browsing 64bit or otherwise without the requirement of the AOS add-on by using the Web Shield.
Personally I don’t feel that the AOS add-on brings much to the party, the real of security of browsing (detecting/blocking malware/malicious sites, etc. comes from the Web Shield