Tweakguides have a large article about firefox tweaking which not only explains the basic settings of firefox but also advanced settings which can be edited by entering about:config in the firefox address bar. The guide is divided into 12 chapters and starts with an introduction which helps you get in the right mood for the article. After that the optimization chapters begin with chapter 2 essential optimization and an explanation of many basic firefox settings. You learn a lot about firefox and those early chapters are clearly aimed at novice users who did not put much thought into the configuration up to now.
The advanced sections begin with chapter 7 (customizing firefox) and explain again many variables that can be altered using the about:config command. This is a great way to tweak firefox without using an extension like fasterfox that automatically alters some variables depending on how you set it up. All chapters include links to additional information, the advanced tweak section for instance provides links to a developer website at mozilla.org that explains the different variables as well. This guide is a great ressource for every firefox user, definatly recommended.

Firefox Tweak Guide