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network.http.pipelining
network.http.pipelining.firstrequest
network.http.pipelining.maxrequests
network.http.proxy.pipelining
Just entering "pipe" in the search bar of about:config should bring up those four options. Set the first two and the last one to be true; set maxrequests to be a high number (like 32 or something).
I've also adjusted nglayout.ititialpaint.delay to 0, which also makes a big difference in rendering speed (though it can also cause some minor, irregular rendering problems on some pages).
Adjusting those settings will really pick up your speed with FireFox, but there are also other little tweaks here and there that seem to help as well. But, those seem to vary according to your machine and connection speed. The mozillazine forum thread "Tuning FireFox" has many of the specifics; I don't know any real details myself.
<edit>A new profile from time to time also really helps, particularly if you've goofed one up (which I've done several times). I install all extensions to my profile. That way, if one turns out to be undesirable, I only have to create a new profile instead of doing a completely new install. Also, I've always gotten much better results with the zipped versions than the installers.</edit>