On Macintosh OS 10.4, Firefox not only takes roughly 5-10x as long to load as Safari but eats up resources substantially. At one point, when not loading any videos and with 4 tabs open, Firefox was eating 40-60% of cpu time and my fans were blaring. One minute after closing it, the machine's responsiveness picked up and the fans slowed to a near halt.
On Gentoo, Konqueror also loads significantly faster than Firefox, and Firefox is there also a proc/memory hog.
Recently I tested Firefox 3 beta on Kubuntu Hardy and I have to say, it's not getting any faster and actually I can notice a further slowdown. If Mozilla doesn't do something soon, it might find itself losing favor in the community.