looks like you are using saferfox Epanded as your Firefox theme. Tyr defaulting your theme to firefox default. Also, if you are launching firefox from inside another application you may get this display problem. for example, when you "preview in browser" inside of Dreamweaver, sometimes you get this problem. Aside from that, I wouldn;t know about the Traktor DJ problem with the skin except to say that some software is written to specifically display a certain way, regardless of the style you have chosen, for example, iTunes does not skin, as the interface is written to specifically adhere to it's own theme.
