My “automatically check for updates to... Firefox” is grayed out (running unpriveleged on Ubuntu). It is however checked, so presumably updates will be checked for. The other two “Installed Add-ons” and “Search Engines” were not grayed out, so I unchecked them (I want as few update checking thingies as possible).
Firefox updates for Ubuntu come through the update manager, which comes from Canonical. If you check your auto-updates through “about:config”, what does it show there? Mine shows True, but the updates always come from the update manager.