Firefox extensions

Dev Tools

1. Web Developer
2. View Source Chart - colorful, table view of source
3. View formatted source - inline viewing of selected elements in pages
4. ColorZilla - work with colors
5. MeasureIt - ruler for measuring pixels
6. JSView - view javascript source for external files
7. HTML Validator - based on tidy
8. Venkman JS Debugger - useful!
9. Clear Cache Button - for your toolbar
10. LiveHTTPHeaders - watch HTTP headers while the page loads
11. User Agent Switcher
12. Link Checker
13. Screen Grab - creates a screenshot of scrollable pages

UI

1. FasterFox - speed tweaks
2. Well Rounded - creates round ui elements
3. TabMixPlus - tab enhancements
4. All in one Gestures
5. Colorful Tabs - fancy colors for tabs
6. Download Statusbar - better than the download manager
7. Download Them All - download manager
8. Image Zoom - zoom in/out for images.
9. Resize search box resizes the search box
10. IETab - for those web pages which just refuse to work with anything except for IEeeeeeeee!
11. AutoCopy - select text to copy it automatically ala unix
12. Search Plugin Hacks - adds functionality to the search plugin at the top next to the address bar
13. FoxPose - mac os expose like functionality for the firefox tabs
14. FlashBlock - block flash animations unless you enable them

Ad Blocker

1. AdBlock - no more ads!
2. AdBlock Filterset - filters for ads
3. AdBlock Plus - adds to AdBlock functionality.

Misc

1. ForecastFox - weather on the statusbar
2. Wizz RSS Reader - decent RSS reader
3. PDF Download - lets you download pdf files instead of rendering them in a tab if you want to.
4. GMail notifier
5. MR Tech Local Install - manage local installations of extensions and themes
6. Video Downloader - downloads the videos on youtube, google video, etc.
7. Torrent Search Bar - search for a torrent across torrent search engines

See this for a nice guide for firefox which lists some of the extensions I find useful along with other useful info about firefox.

enjoy!