Do a search here for "Firefox" or "extensions" or some other such thing. There's at least 2 threads here for "gotta-have" extensions that folks list.
As for why I use it, security is the biggest thing. I even go to the extent of installing it on anyone's PC I tend to work on (the in-laws, mainly), deleting all commonly found traces of IE, and changing the Firefox icon to IE's to make them THINK they're using Internet Explorer. There's only one person (brother-in-law) who has something he NEEDS to use IE for, and it's his bank. It uses some ActiveX thing, which Firefox doesn't allow, as far as I know. He also ONLY uses IE for banking, and I showed him how to browse to it within Windows Explorer so he doesn't have to unhide it - and he doesn't.
I started using it mainly because of security and tabbed browsing, but like PDB, I choose to stick with it due to customization. Don't just check out the extensions, check out the Themes as well. I use the MiniFoxFlat for older monitors that I wish to have more screen real estate with, and Noia 2.0 (extreme) for anything else. There's a fistful of extensions - so many that it's worth your time to set aside a bit of time and just sift through them and test them out. I currently use:
Bookmark Duplicate Detector
BugMeNot
Colorful Tabs (different COLORED tabs - I often have a dozen or more tabs)
Dictionary Search
FlashGot
Gmail Manager
MR Tech Disable XPI Install Delay
PDF Download
Sort Extensions and Themes
Super DragAndGo
Text size toolbar
TinyURL Creator (although really only used for lemon disguises

)
I'm sure you can find tons of extensions you'll like. FasterFox will soon eliminate the "delay" you perceive, for one. Just remember, install one extension at a time. They CAN be finicky and cause problems with each other.