This also highlights an interesting network effect. When a browser has high market share --- especially among Web developers --- developers will optimize for that browser, working around areas where it's slow and taking advantage of features which are fast. Browsers that are trying to break into the market don't get that benefit so they have to try to be faster in all areas where the dominant browser(s) are fast. This is a considerable burden. Hats off to Opera and Safari for doing such a great job.
Of course, even if developers are working around a performance problem in Firefox, we still need to address the problem ... not just to keep up with the competition, but to make life easier for Web developers. And sometimes our performance problems just stop people from using a feature of the Web platform, and we definitely need to avoid that.