If this were a significant problem, the Mozilla forum would be burning up with this question; since it isn't, the problem is somewhere on your end. I found one person who had the same error message as you (in
this thread); the recommendation was to uninstall the extension "User Agent Switcher", which worked for at least one person. A second possibility is that you are using the extension Firesomething to change your useragent; if so, undo that part of Firesomething.