One alternative would be for us to switch to using Icecat as the default browser, possibly modifying the .desktop file to just report itself as "Browser". It is fundamentally the same browser as Firefox, except not bound by the "you cannot change anything and still call it Firefox" rules.
But our users expect Firefox, not Icecat. Besides doesn't Icecat forbids you to install non-free extensions? So with that move we would actually exclude more extensions from our users than shipping Firefox.