I don't run xinemera, but I would think that maybe you should look instead on problems with your windows manager (which you didn't mention). Most WM's are still only beginning to incorporate xinemera support (if they've even noticed it), and i would presume that mozilla has nothing to do with it....its just another window that should hopefully be handled correctly by the wm (w/ xinemera). I could of course be very wrong... just shooting around ideas.
*Maybe its b/c you handle mozilla/galeon fullscreen (?) and not any other apps...that you havent noticed this w/ other things...again pointing to WM. I, again, have no idea what I'm talking about.
/me is a little slow...xinerama, not xinemera.