you will be able to run "random" extension if the developer care enough about it and about the new security procedures to sign it. After all, it takes only couple seconds for the signing to work.
The versions I quoted are not non-standard. They are all versions of Firefox being worked on and with all the relevant teams. All those versions eventually become Firefox Stable and after that becomes outdated and a new release is now current. Versions goes from Nightly -> DevEdition -> Beta -> Stable. Each version has some tweaks, for example DevEdition is where they seed and test new devtools. Which means that for the developers, thats the best edition to develop with (still test on the other versions).
Do you understand that the Unbranded Stable version and Firefox Stable version have the same codebase? You can use that version for testing or if your users don't want signing they can move to that version. They lose the cute icon and branding but the code is the same.
I think you missed my very clear point: now it's not enough to just run Firefox. You need to ask for users to run the "right" version if Firefox.
Telling people what browser to use is user hostile behaviour. Users will not bother. Non-official extensions will get less interest. Authors will see a smaller user base and have less interest in writing new extensions.
The versions I quoted are not non-standard. They are all versions of Firefox being worked on and with all the relevant teams. All those versions eventually become Firefox Stable and after that becomes outdated and a new release is now current. Versions goes from Nightly -> DevEdition -> Beta -> Stable. Each version has some tweaks, for example DevEdition is where they seed and test new devtools. Which means that for the developers, thats the best edition to develop with (still test on the other versions).