Every time I use Firefox on one of my Macs I'm reminded what a miserable hack their OS X integration is. The controls don't quite feel right, text selection highlighting always uses blue. The worst feature is the menus though. Context menus are a joke. They've got square corners where the native OS X menus are rounded. The icing on the cake though is being able to have multiple context menus visible at once. In fact it's almost impossible NOT to have multiple context menus open.
Right click on something outside of Firefox, and then right click somewhere inside Firefox. Stunning.