I’m not sure if this is of any help but I’m using Firefox 29 and Avast Free 2014.9.0.2018. Both are on default settings and appear to be working without any problems.

The only thing that has changed recently is that the Avast Online Security page doesn’t automatically appear when Firefox loads. Perhaps it was cancelled by yesterday’s upate?

Oops … spoke too soon! Now, I too am getting ‘Unknown Site’ from AOS on this webpage!!!

Very odd … now it has gone back to normal again after logging out then logging back in to Avast with a restart of Firefox.