One of the nice things I like about Firefox is that it's almost infinately customizable as long as you know how. You can create your own themes, modify how buttons look, create your own extensions and such. A lot of a extensions are just tweaks some guy did to his configuration file and then thought it was cool enough to share with others.
Then again a lot of extensions drasticly change/add functionality that is usefull for things for various purposes. For instance there are developer aids for webmasters to check syntax and proper form that you can add into firefox.
One big thing I don't understand to much about is CSS style sheets. But you know how to hack around that stuff both you can create style sheets to override defaults on webpages, and even change how the UI itself looks and feels.
The more you know the more you can hack at it and modify it, even all the way down to the source code itself if your so inclined and have the skill. They try to make it relatively easy and the sky is the limit. (after all this is a browser for hackers to get a browser as close to their ideals as possible (meaning dedicated programmers)).
It's a complete 180 from what most people are familar with from using MS products forever and ever.
check out here for some details
(especially be sure to check out the tips and tricks sections for some very interesting and fun hacks.)