Im sure I haven't used any masking tools for firefox, because I have also installed internet explorer for the rest of my family, which is prefered by them. Seeing this error was quite a surprise to me, I even thought this is some kind of joke from rms :p
So anyway I will continue to find out the cause of this malfunction.
edit. "User Agent Switcher" seem to work, so the problem is kind of solved, but still don't know why would that error appear.
ps. I just installed that user agent switcher, so its not like i had it earlier.