This would not be a problem if Mozilla had sided with users for once and allowed us to install xpi extensions from outside their app store.
Becuase if they're not I can't imagine not being able to side-load them.
And if they are, with the amount of Firefox forks we have, it should be trivial to create one that allows for side-loading unsigned or 3rd party signed extensions. Hopefully without too much source modification, just creating a distribution of Firefox would easiest to manage.