Once you are familiar with the procedures for changing these settings to optimise Firefox, go to this site for even more detailed modifications. The guy here shows how to optimise Firefox depending on your connection speed and computer speed.
The site is at
http://codebetter.com/blogs/darrell.norton/archive/2005/01/28/48720.aspx
He provides specific settings depending on whether you have:
A Fast Computer & Fast Connection;
a Fast Computer & Slower Connection;
a Fast Computer & Slow Connection;
or a Slow Computer & Fast Connection.