Well FF 40.0.2 being the latest regular release of firefox and whilst that flags AOS as not signed, it isn't stopping it on my win7 system (I don't have it installed on this XP system).
For now it is possible to enable it within the FF add-ons, come FF 41.0 there will be further changes, but they can be over written in the about:config.
Hopefully by that time the AOS add-on will have been signed and approved.
FF 42.0 will see a further tightening on permitted add-ons.