The majority of the information comes from
this excellent thread over at the MozillaZine Forum.
First, install
Chromedit extension. It make it easier to edit user files.
Now, run Firefox, go to Tools > Edit User Files, click user.js tab.
Insert the following texts based on your PC speed and internet connection.
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

lugins and the bookmark menu delay is turned off.
Code:
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
Code:
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.
Code:
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
Code:
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
Code:
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
Code:
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);
Click Save and restart Firefox
Happy surfing..
-------------------------------------------------------
another speed tweak..
Open Firefox.. type about:config
look for network.dns.disableIPv6
Double click to toggle it to 'true' and then restart Firefox. You should notice a speed increase, page to page.
This tweak disabe IPv6 in Firefox, because most if not all of the web is still using IPv4. IPv4 and v6 are internet protocols... how the data is transferred... v4 is the standard now and v6 is slowly being implemented around the world... it wont come into full swing until 2008-2010 sadly... it's a huge infrastructure change, but the benefits will be great.
Source