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.