I worked around this with a script that patches the omni.ja zipfile to change the setting to require signing.
It needs to be unpatched for updates - I don't allow my normal day-to-day Firefox instance to write to it's own binary anyway. Firefox tells me when there is an update, and I then restart into my special script that unpatches, runs Firefox with permissions to modify itself and a profile that I don't use for day to day browsing, only for updates, and then patches it again.
Developer edition has updates but it's a beta. Unbranded builds aren't a beta but don't get updates. There is no version that's just like normal firefox except without signing enforcement.
Developer edition and nighties are not release builds.
Unbranded builds don't get updates.