If you are like me, who spends most of his time online on the browser, Firefox can be a bit of a drag. It seems like it is going worse with each new release. Part of the problem is the number of memory hogging plugins you use but even on a fresh Firefox install it seems slower. Which is why I am anxiously waiting for Chrome to be fully ported to Linux.
Swiftweasel is a project that makes processor specific builds with the highest level of optimization as well as P.G.O (Profile Guided Optimization), which results in a faster and more responsive browsing experience. Specifically, my personal observation was that cold startup is significantly faster. You should be able to install all Firefox plugins and it has four plugins pre-installed for you: AdBlock Plus, XForms, User Agent Switcher and Quick Locale Switcher.
There is a similar project called SwiftFox, which doesn’t use PGO optimization and has proprietary license for its binaries, but it is easier to install. Definitely worth a look.