The X for closing tabs is right-oriented; how can I make it left-oriented?

I'm on a mac. My x'es are all upper-left. When I recently made the swap from safari to firefox, I noticed that the tabs in firefox all have their crosses in the upper-right corner, whereas the browser itself and all other windows have upper-left crosses. If there's a quick fix, I'd love to know. Changing my operating system is neither quick nor very attractive.