It's so strange to hear people say this. I'm running FF 3.6 on Ubuntu 10.04 x64, and its performance is comparable to Chrome. Chrome's a tad faster, but not significantly so, while FF is actually more stable with tons of windows open (~45+). Flash crashes about equally in both (and worse in Opera 10), although not so frequently as to be unusable.

FF is actually so good I switched from Chrome/Chromium to FF a few months ago (mainly for Vimperator though, but the performance differences were marginal enough not to matter). I can't help but wonder if 64bit Linux is FF's optimal platform, with its performance dropping off on Windows and maybe Mac? What OS are you guys running it on?