It does? But I'm using add ons that I made on Chrome that I haven't registered or signed anywhere. I thought Firefox disallows that?

Signing is pretty important. Without signing anybody could surreptitiously replace your addon with something else.

How so? Either they already have access to my computer, in which case browser addons are the least of my concerns, or they've compromised Mozilla in some way in which case who's to say they can't sign their fake addon too?

But it also means that I can't have an add on that Mozilla doesn't sign, right?

Only if they have local access. In which case, I’ve already lost.