When I put ubuntu on my machine at home firefox 3 seemed substantially slower than it did on windows. I'm not sure if FF3 is crappy on iinux or if there was something about my install that was screwed up.
Firefox has some serious issues on Linux. It's admirable that they make a browser for Linux, but they do seem to let a lot more fall through the cracks than on other platforms.
For example, I've seen several websites that crash Linux in Firefox -- not good. The rendering in general is also not as good as win/mac, esp. with respect to transparency.
I have 10,000RPM drives in RAID 0, so my computer is always faster to open a local app than to open a webpage.