Well. As you can see, the position uses the attribute from game.css
At this point I would use standard bug search pattern.
Make backup of your userContent.css and try naked file with only the absolute important setting.
If this fails, try -p to start firefox with profile manager and make a fresh profile to try out. My best guess is, that you have some interfering addons or a } problem with your userContent.css. Syntax highlighting editors are good for that.