I think it's mainly a difference in configuration. The developer version, IIRC, allows you to install extensions on your own machine without seeking approval from Mozilla, but on the other hand it apparently subjects you to additional telemetry:
> Firefox Developer Edition automatically sends feedback to Mozilla ... In addition to the data collection described in this Privacy Notice, these versions by default may send certain types of web activity and crash data to Mozilla and in some cases to our partners.
If you just want to be able to install unsigned extensions with neither the extra bugs from running a beta nor the telemetry, you can use the far less advertised "unbranded builds" of Firefox [1] instead.
It's possible. Last I heard was that the unbranded builds were too eager to update [1] so it could well have been disabled totally. The Mozilla wiki doesn't mention anything about updating being disabled which to me implies it is not. If you find out for certain, please let me know!
I don't personally use the builds as Mozilla has not yet blocked signing of our extensions and `web-ext run` serves me well for development. Automated signing is not brilliantly documented but works well enough [2-3] to ensure that our builds are tested by real users before we release a stable version.
When was the last time devs took time out of their day to create detailed bug reports with crash logs and context? Telemetry done right is just a way to surface bugs properly.
Note specifically the step where the addon must be signed before it can be installed in this fashion. "Developer edition" does not require you have Mozilla's permission to install addons.
Special features: kind of like an IDE on top of Firefox, meant to be used by web developers. Comes with its own config profile, so you can fuck around and not break stuff in stable/beta that you are supposed to have concurrently installed.
Kind of like beta: all of the new APIs and features related to web development are there three months before they hit stable.
Seems very much a beta for developers. Some cool dev features promised, but didn't always work as demoed. Font parameter manipulation for example. UI's there and pretty...it just doesn't affect the look/styling of fonts rendered on a real site like it does their demo site. Also, a number of UI controls slow to respond (e.g. cursors for dragging, clicking, etc.) β β β so very β
> Firefox Developer Edition automatically sends feedback to Mozilla ... In addition to the data collection described in this Privacy Notice, these versions by default may send certain types of web activity and crash data to Mozilla and in some cases to our partners.