The Firefox Tweak Guide has the full details on how to speed up Firefox regardless of your connection or hardware, reprinted in part below. Don’t forget that the easiest way to tweak user files is with chromEdit extension.
Common to all configurations
These are the settings that seem to be common to all configuration files regardless of connection speed or computer speed with a couple of additions – plugin paths can be found with about:plugins and the bookmark menu delay is turned off.
user_pref("network.http.pipelining", true);
user_pref("network.http.proxy.pipelining", true);
user_pref("network.http.pipelining.maxrequests", 8);
user_pref("content.notify.backoffcount", 5);
user_pref("plugin.expose_full_path", true);
user_pref("ui.submenuDelay", 0);
Fast Computer Fast Connection
user_pref("content.interrupt.parsing", true);
user_pref("content.max.tokenizing.time", 2250000);
user_pref("content.notify.interval", 750000);
user_pref("content.notify.ontimer", true);
user_pref("content.switch.threshold", 750000);
user_pref("nglayout.initialpaint.delay", 0);
user_pref("network.http.max-connections", 48);
user_pref("network.http.max-connections-per-server", 16);
user_pref("network.http.max-persistent-connections-per-proxy", 16);
user_pref("network.http.max-persistent-connections-per-server", 8);
user_pref("browser.cache.memory.capacity", 65536);
A couple settings of note – Firefox is allocated 4096 KB of memory by default and in this configuration we give it roughly 65MB as denoted by the last line. This can be changed according to what is used.
Fast Computer, Slower Connection
This configuration is more suited to people without ultra fast connections. We are not talking about dial up connections but slower DSL / Cable connections.
user_pref("content.max.tokenizing.time", 2250000);
user_pref("content.notify.interval", 750000);
user_pref("content.notify.ontimer", true);
user_pref("content.switch.threshold", 750000);
user_pref("network.http.max-connections", 48);
user_pref("network.http.max-connections-per-server", 16);
user_pref("network.http.max-persistent-connections-per-proxy", 16);
user_pref("network.http.max-persistent-connections-per-server", 8);
user_pref("nglayout.initialpaint.delay", 0);
user_pref("browser.cache.memory.capacity", 65536);
Fast Computer, Slow Connection
user_pref("browser.xul.error_pages.enabled", true);
user_pref("content.interrupt.parsing", true);
user_pref("content.max.tokenizing.time", 3000000);
user_pref("content.maxtextrun", 8191);
user_pref("content.notify.interval", 750000);
user_pref("content.notify.ontimer", true);
user_pref("content.switch.threshold", 750000);
user_pref("network.http.max-connections", 32);
user_pref("network.http.max-connections-per-server", 8);
user_pref("network.http.max-persistent-connections-per-proxy", 8);
user_pref("network.http.max-persistent-connections-per-server", 4);
user_pref("nglayout.initialpaint.delay", 0);
user_pref("browser.cache.memory.capacity", 65536);
Slow Computer, Fast Connection
user_pref("content.max.tokenizing.time", 3000000);
user_pref("content.notify.backoffcount", 5);
user_pref("content.notify.interval", 1000000);
user_pref("content.notify.ontimer", true);
user_pref("content.switch.threshold", 1000000);
user_pref("content.maxtextrun", 4095);
user_pref("nglayout.initialpaint.delay", 1000);
user_pref("network.http.max-connections", 48);
user_pref("network.http.max-connections-per-server", 16);
user_pref("network.http.max-persistent-connections-per-proxy", 16);
user_pref("network.http.max-persistent-connections-per-server", 8);
user_pref("dom.disable_window_status_change", true);
One of the changes made for this particular configuration is the final line where the status bar is disabled for changing web pages to save processor time.
Slow Computer, Slow Connection
We have entered the doldrums of the dial-up user
user_pref("content.max.tokenizing.time", 2250000);
user_pref("content.notify.interval", 750000);
user_pref("content.notify.ontimer", true);
user_pref("content.switch.threshold", 750000);
user_pref("nglayout.initialpaint.delay", 750);
user_pref("network.http.max-connections", 32);
user_pref("network.http.max-connections-per-server", 8);
user_pref("network.http.max-persistent-connections-per-proxy", 8);
user_pref("network.http.max-persistent-connections-per-server", 4);
user_pref("dom.disable_window_status_change", true);