Your experience is not universal. FF generally runs well on Ubuntu. However,
many add-ons can slow it down. You can also install FF's betas which give
better performance. Most people that I talk to find that Google's Chrome or
Chromium runs faster than FF. You can also give Opera a try. It is quite
fast. I am running FF3.5 and the betas and all are fast on my system. It is
difficult to make comparisons between OSes and distros. There are too many
things that could account for any perceived difference. It could be due to
the OS itself or the compiled offering that you are using and whatever you
have running at the time, including different add-ons. You could also try
going naked... that is, running it without any add-ons.