I'm not involved with Mozilla anymore, but last I heard, there are things you have to use Developer Edition for that can't be done with vanilla releases (like working on add-ons).
It is possible, you've just chosen to disallow it.
To be clear: the Firefox team took deliberate steps to make sure that add-ons can't be tested or developed in the stock builds of Firefox downloaded from Mozilla.
See your sibling below: > Unsigned addons cannot be enabled on beta or release. That was only available for a limited time. Now they can only be enabled on alpha (aka developer), nightly, or special unbranded builds.
You can make a userscript without any of that hassle, though.