Change for the sake of change isn't necessarily improvement
I've been a Firefox user for a long time, following a few years of using Netscape back in the 90s. I agree with a previous poster that it's more important and more beneficial to fix performance issues first. How about addressing the fact that Firefox doesn't always render pages correctly, and then we are forced to open the page in another browser, i.e. IE?
Fix the engine, and let the FF community introduce new Themes and Personas to allow users to change the look of the browser ... if they CHOOSE to.