Let’s face it, Firefox isn’t exactly the fastest browser around. In Linux it’s worse, in fact, Windows Firefox through Wine in Linux is faster than Linux Firefox.
Tuxradar benchmark Firefox 3.5 Beta 4’s performance and the results are very optimistic.
I use Firefox and Opera in Ubuntu and I prefer Opera in terms of speed but it just never felt quite like a native application in Ubuntu. Firefox just doesn’t appear as responsive sometimes.