Firefox 16.0.1 - Buttons have a weird-acting line

Created by: devfrey

I'm running Firefox 16.0.1, and I get the following issue: http://puu.sh/1eJ0U Basically a random line appears on .btn elements (not always, but when it appears it keeps re-appearing) And when you hover the button, the line will shift to the center which looks really ugly: http://puu.sh/1eJ2p