No, Fx 4.0.1 and Fx 10.0.7 Enterprise do NOT support this protocol.
Sea Monkey, older version 2.6.1, does NOT support this protocol. However, SM current version 2.12.1 does and has a bunch of entries in about:config.
So this protocol was added to Mozilla browsers within the last six months thereabouts. Just another reason to avoid the insanity of the current "race to death" frantic, constant updating of browsers. I don't see limitations in Fx 4, or 10, or the earlier SM, because they don't have this "feature". At least, I only have to disable in SM 2.12.1.
I haven't checked Opera 12 yet.
When governments fear people, there is liberty. When the people fear the government, there is tyranny. Thomas Jefferson