Mozilla is in a tough spot as a browser vendor, since a sizable portion of their userbase is actually well served by the restrictions. Happily it's also open source, allowing true power users to do what they want with minimal fuss.
Most people can live with the restrictions you impose on them, but the restrictions can still be missguided or thoughtless.
I think one of the issues is that users aren't the only ones that can install extensions locally. I think Mozilla even introduced this as a response to Microsoft pushing its own extensions into Firefox.
I don't like the restriction either, however most users are dealing with an actively hostile OS and any setting that could be set at runtime instead of compile time would just look like an open invitation to the OS.
For what it's worth (admittedly not very much), you can build the release version of Firefox yourself while allowing extension sideloading without even changing any code, it's just the "MOZ_REQUIRE_SIGNING" option.
Obviously this is not a real possibility for everyone, but I do it for this and getting rid of Pocket, along with a few other small changes.