One of the largest reasons I use [http://getfirefox.com Mozilla Firefox] is its huge [https://addons.mozilla.org universe of extensions], scripts, and other add-ons to it behave the way you want it to. I've customized my Firefox installation pretty extensively.
This list is current as of Firefox 3.0.
[https://addons.mozilla.org/en-US/firefox/addon/60 Web Developer]
[https://addons.mozilla.org/en-US/firefox/addon/3615 Delicious Bookmarks]
[https://addons.mozilla.org/en-US/firefox/addon/2324 Session Manager]