FF is often bloated with Add-ons which Chrome/Chromium has fewer of. As you
update your add-ons they can add to the bloat which is out of Mozilla's
control as they do not make most add-ons. The only way to make a fair
comparison is to run each in its virgin state. Tests on FF 4 make it faster
than current Chromium, but this is ever changing because Chromium is
improving too.

FF 3.x is older than Chromium for the most part because Google releases
things differently. Mozilla tests them and releases them only when ready.
Everything with Google is usually in some stage of testing when you get your
hands on it. There's is a perpetual beta that it designates as stable or
unstable, but never as final. This is not saying Google's product is less
stable, just that they do things differently. By the time most users get
their hands on the next FF, it has been months in the hands of testers and
we keep hearing about it but it takes time to make it into the repos (unless
you use a PPA or download and install the testing version). Google seems to
leapfrog Mozilla because it seems to turn things around much more quickly.

Ubuntu 11.04 played with the idea of using Chromium as the default browser,
but stuck with FF because the next version is said to be much faster, but
then most users usually slow it down with add-ons. For me it comes down not
to speed, but convenience. I find Chromium too streamlined for my way of
working. I really miss not having the ability to skip back through web pages
I have visited. I can do this from the FF dropdown but Chromium makes me
click back through each a page a time because it has no dropdown list. There
is a perceptible difference in speed, but not enough that I worry about it.

There are ways to speed up FF. You can Google it. It involves typing
about:config in the location window and making manual changes or an easier
way is to install Ailurus and it will make changes with one click of the
button. It is like Ubuntu Tweak. http://code.google.com/p/ailurus/