One easy, cool preference you might want to tweak deals with speed. By default, Firefox runs fast enough, it's like driving at the speed limit. But if you want to push your browsing experience, modify a few settings in the 'about:config' file. Note that these edits are for broadband connections only; if you're still dialing up, you're stuck with whatever speed you've got.
Type in 'about:config' in the Address bar and press Enter. On this screen, scroll down to the 'network.http.pipelining' options. The name says it all: Firefox normally processes all HTTP requests sequentially. Once it sends one out, it waits for a response before sending the next request to a server. If you set 'network.http.pipelining' and 'network.http .proxy.pipelining' to true then you've basically just upgraded an antiquated browsing system with modern plumbing. Set the 'network.http.pipelining.maxrequests' option to 30 and Firefox will now send up to 30 HTTP requests at once, theoretically, although not always, making your browsing that little bit faster.
Before you close the window, right-click anywhere on the 'about:config' page and select 'New->Integer'. Then type in 'nglayout.initialpaint.delay' and enter '0' as its value. Firefox will now render pages immediately instead of waiting its default 250 ms, a slight but noticeable difference when you're surfing the Web.