You're welcome
This is only related to FireFox.
Bob is correct.
Essentially, when it updates it checks the install.rdf file of each addon and if the Firefox version is higher than listed in that file, it reports it as incompatible. This is done regardless of whether the addon works or not.
The decision to change the avast one to have no maximum version listed means that Firefox will never just report that it is incompatible. It is easier this way, since avast would have to release a program update just for WebRep each time Firefox updates, so that it would work rather than have the forum (etc.) filled with posts about it needing updating.
So far, this has been fine and the changes to FF have not had any impact on the functioning of WebRep. Naturally though, there is the possibility that a future update will cause issues.