Whilst avast is a 32bit application, it has the 64bit drivers, etc. when installed on a 64bit OS.

So I don’t think that is it but ‘Gopher John’ asked some questions in his first reply that weren’t answered, notably these.

Does WebRep show as installed for Firefox in the AvastUI? Does WebRep show in the Firefox Add-ons tab | Extensions? If it does, you may have to enable it there?

There is a possibility that waterfox 64bit browser may be the issue, even though it uses the same firefox stuff, I think it significantly different not to be considered Firefox and as such the web rep isn’t compatible. only with IE, Firefox and Chrome currently, so that may be it.

That is were first question in the quote from Gopher John above is very relevant.