With the latest version of Firefox, they've disabled the Avast browser addon because it can't or isn't verified.
this is the reason they give. https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox?as=u&utm_source=inproduct
Is there an ETA on an update?
You say you have the latest version but you don't say that is regular, beta, alpha or nightly, etc.
I have FF 40.0.2, normally I don't run the browser add-on, but I have just dons a change and included the Avast Online Security (AOS) add-on. FF had a bit of a whinge when installing an add-on not from the mozilla source, but I was able to accept it.
On rebooting FF looking at the add-ons the AOS add-on isn't disabled, it is however flagged as unsigned, but it is running and displaying the AOS icon at the top of the pages I visit.