One of the largest reasons I use Mozilla Firefox is its huge 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.
Extensions
Web development
Other stuff
- Automatic Save Folder
- Colt
- FireGPG
- Fission
- Google Gears
- Grab and Drag
- Link Widgets
- Mobile Barcoder
- Multiproxy Switch
- No Squint
- Open in Browser
- Read it Later
- Selenium IDE (disabled)
- Show Keywords
- Tabs Open Relative
VeriSign's OpenID Seatbelt
- YSlow