Just saying, "anyone can write code, be careful" gets you out of a lot of trouble. Saying "We've checked these and they are good" buys you a lot of headaches. That's the first problem. Who's going to test the extensions? Who's going to be liable when a "tested" extension is malware? It WILL happen, you know it. Who is going to maintain the cert?
No user work-around? That's pure insanity. What happens when a vendor says "This is too much trouble, we can afford to support firefox anymore," their customers will have to switch browsers.
Lastly, having any group of people dictating what others can do is against the whole notion of free and open source software. I have absolutely no problem popping up a dialog that says, "This extension has not been tested by the Mozilla Organization, Proceed at your own risk," but not even having that option is totally and completely bogus.
Time to fork.