It does not look like a desktop environment problem, but more about Firefox or about Facebook.
I have put your message error in google and in ixquick.com, just this part:
[Exception... "Component returned failure code: 0x8000ffff (NS_ERROR_UNEXPECTED) [nsIPrefBranch.getComplexValue]"
I don't have a Facebook acount. Would you take a look here once logged in there ?http://ro-ro.facebook.com/Socialfixer
maybe they have something about it, not sure (never know with search engines results).
Other things to try : start firefox in console with "--safe-mode" option, to temporarily deactivate your plugins.
One more : try moving your ~/.mozilla directory out of the way (rename it to .mozilla-back temporarily) and start Firefox again.
One more : try login to Facebook with other browsers : Opera, Midori...
And if at the end nothing works, would you go to the #firefox chan to ask advise from the specialists ?
the chan is #firefox and the server is irc.mozilla.orghttps://wiki.mozilla.org/IRChttp://irc.mozilla.org/
I hope one of the above will help.