I understand about the 'safety issue', but I think people are more mad about their extensions disappearing than they were about the 'safety' of the extensions themselves.
In my opinion, a better way to implement the signature extensions would have been to give users a pop-up or something that said:
"The following extensions do not have a signature from Mozilla. [List user's extensions that don't have a signature.] This means that they may be more unstable or they may not be safe (or as safe) as using an extension that has a verified signature. Would you like to continue using these extensions? {Yes} {No} Mozilla is not responsible for any breached data as a result for using unverified extensions, blah, blah, blah, disclaimer stuffs."
That way Mozilla wouldn't have to be concerned that they would get in trouble for whatever it is that they are worried about, and people could still have their extensions.