Firefox has a great user interface. It’s come a long way and a lot of very talented designers have worked to bring it to a place of visual and interaction stability. So, the question of how to improve becomes more difficult, but also more interesting.
This is an observation more than a criticism, but have you ever noticed how many redundancies occur in Firefox? Chrome’s tack has been to eliminate some of these and scrap the interface down to the bare minimum, which is probably going too far, but perhaps there are ways to clean the UI in places without sacrificing functionality or usability.