I can murder some trees and poison the environment for all of us, to do pointless mutli-hour re-builds of Firefox for each release and point release to have it accept my add-ons, though.
I've also never seen a reason, why I can't at least place my CA into Firefox /usr/lib/firefox folder or /etc/firefox and have it be respected. Or just place local extensions there and have firefox not require signatures for them, because there's no way these can be installed accidentally from web by clicking some link.
And if someone can trick me into modifying /usr/lib, they can just trick me into replacing Firefox completly with their malwared build, so signing will not save me anyway.