Make sure Mozilla is completely closed. Using your operating system's search feature, look for the following file:

prefs.js

Open prefs.js in your text editor (Notepad, TextPad, whatever ... NOT Word). Scroll down (everything is in alphabetical order) until you see the appropriate place to add the following lines:

user_pref("network.http.max-connections", 128);
user_pref("network.http.max-connections-per-server", 48);
user_pref("network.http.max-persistent-connections-per-proxy", 24);
user_pref("network.http.max-persistent-connections-per-server", 12);

Save and close prefs.js. You may need to restart Mozilla. You should now notice markedly faster page loads, especially if you are using a fast connection, such as DSL or cable.

One note: your pages will definitely load faster, but you are really stressing the servers from which you are requesting pages. Therefore, you may want to lower the numbers above, or only use this trick when you really need it.