Mozilla has 64-bit firefox in its ftp server for download for months (I don't remember starting from which version).
She kept all versions as beta and never release the final...just the final

Avast add-on has worked fine with 32-bits firefox but not with 64-bit as I known and tried myself.
Today, Mozilla released a candidate copy of 64-bit version 42 and they have never did before.
So, I think Mozilla is going to start release offical copy of 64-bit firefox.
If my experience with 64-bit version is correct, then Avast has to do something......