Firefox has seemed sluggish on my Ubuntu installation, which I attributed to my aging 1.1Ghz PIII, and rather limited memory (512MB). In a fit of frustration, I googled for “speed up Firefox” and came up with some good tweaks to the config that made a huge difference. If you are a Firefox user, you owe it to yourself to check this out.
Link 1 - This is a good start, and if you do nothing else, make these changes.
Link 2 - This is a little more in-depth. It contains the same basic tweaks as the first article, plus several others. It also has the tweaks grouped by whether you have a fast/slow computer and a fast/slow connection.
Hope this helps.