I did manage to solve it in the end, although I still have no idea why the inline code - which was added by Firefox, not me - was added.
I compared the html/css class code for the menu with another menu that wasn't doing the same thing. This code was faulty:
<?php if ($this->countModules('mainmenu')) : ?>
I'm not sure if i got it right but you can try changing the core files of the menu and try to remove the inline css code (if that is causing the problem). You can yet add media queries to your css file to verify the screen size and apply different translate 3D effects (or any other style) to your menu.
The core files of your menu usually lie in modules\...