What's That Preference will tell you what they are. You may not realize it, but Mozilla actually has loads of information about the preferences in about:config over at the MozillaZine Wiki.

Enhance your web browsing experience with these Mozilla Firefox about:config tweaks. Customize it, improve performance, change settings, add features, etc.

Jack Wallen walks you through the process of locking down options with Firefox's about:config page to help secure the browser from user tampering.

This preference listing is also a form. Right-click on any preference to modify it or to state a new preference. Shorten the display by entering some text in the Filter box if you want. Many Firefox ...