Create a user.js file in your Profile directory for firefox (%APPDATA%\Mozilla\Firefox\Profiles\xxxxxxxx.default\) Dump this following code into that file.
//Speed Tweaks FF3
user_pref("config.trim_on_minimize", true);
user_pref("content.interrupt.parsing", true);
user_pref("content.max.tokenizing.time", 2250000);
user_pref("content.maxtextrun", 8191);
user_pref("content.notify.backoffcount", 5);
user_pref("content.notify.interval", 500000);
user_pref("content.notify.ontimer", true);
user_pref("content.switch.threshold", 250000);
user_pref("network.http.max-connections", 48);
user_pref("network.http.max-connections-per-server", 32);
user_pref("network.http.max-persistent-connections-per-proxy", 20);
user_pref("network.http.max-persistent-connections-per-server", 20);
user_pref("network.http.pipelining", true);
user_pref("network.http.pipelining.firstrequest", true);
user_pref("network.http.proxy.pipelining", true);
user_pref("network.http.pipelining.maxrequests", 8);
user_pref("nglayout.initialpaint.delay", 100);
user_pref("network.http.keep-alive", true);
user_pref("network.http.keep-alive.timeout", 600);
user_pref("network.http.request.max-start-delay", 10);
user_pref("network.dnsCacheExpiration", 3600);
user_pref("network.dnsCacheEntries", 1000);
user_pref("browser.sessionhistory.max_total_viewers", 3);
//set following to false if you find that FireFox is slow to maximize from the tray.
user_pref("config.trim_on_minimize", true);
//security tweeks
user_pref("browser.cache.disk_cache_ssl", false);
user_pref("browser.microsummary.enabled", false);
user_pref("browser.microsummary.updateGenerators", false);
//memory cache Physical RAM Memory Cache (in KB)
//32 MB 2048
//64 MB 4096
//128 MB 6144
//256 MB 10240
//512 MB 14336
//1 GB 18432
//2 GB 24576
//4 GB 30720
//8 GB and up 32768
user_pref("browser.cache.memory.capacity", 18432);
Got this code from a post on DSLReports page, copy here for my own notes.