I've heard that Mozilla have decided to flip the kill bit on the Skype toolbar, an extension for Firefox that is always installed with Skype (whether you want it or not), uses tons of resources and is the cause of an absurd percentage of all Firefox crashes. It makes good sense, since force-installed extensions that reduce stability of Firefox will give the impression that Firefox is bad, where the actual reason is that Skype blows big chunks. Is it a concern? Do people actually want or need this buggy extension? Probably not. Even if you do, you can manually re-enable it, but I think Mozilla have made the right choice in this.
Mokalus of Borg
PS - Now if only they would make it impossible to install third-party extensions permanently against my will.
PPS - I'm looking at you, Java.