Regarding this was reported already for FF4 and it supposed to be browser problem, this is still present in new version, FF5.
http://xenforo.com/community/threads/duplicate-firefox-4-0-1-style-bug.16107/
http://xenforo.com/community/threads/browser-bug-ff4-and-small-css-problem.14032/
http://xenforo.com/community/threads/duplicate-firefox-4-0-1-style-bug.16107/
http://xenforo.com/community/threads/browser-bug-ff4-and-small-css-problem.14032/