Here I’ll just put my most used Firefox add-on list. Actually two lists – one for web development related add-ons and the other for general surfing and enjoying the web.
Developer’s add-ons
- Web Developer
- Firebug
- Firecookie
- YSlow
- iMacros
- PageDiff
- Tamper Data
- Greasemonkey
- DOM Inspector
- InspectThis
Surfing add-ons
- StumbleUpon
- Delicious Bookmarks
- Adblock Plus
- Fire.fm
- Video DownloadHelper
- Cooliris
- Clipmarks
- Rikaichan
- Screengrab
- TwitterFox
There’s also this more official list of add-ons for Firefox categorized by the things you do on the web. So you might want to fashion your Firefox. And of course don’t forget to regularly visit Foxkeh’s Blog for the monthly wallpaper with the Japanese Firefox mascot – Foxkeh 
