- Web developer toolbar
- DOM Inspector (installed with Firefox when you click "custom install")
- Live HTTP Headers
- Adblock Plus
- Add Bookmark Here
- Colorzilla - Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies.
- MeasureIt - Draw out a ruler to get the pixel width and height of any elements on a webpage.
- Nuke Anything - Remove any object from the page.
- Pearl Crescent Page Saver - lets you save an image of a web page to a file in PNG format
- (can also be done from the web dev toolbar)
- Reveal (good example use of
- Tab Mix Plus
- Web Development section of Extensions site
- Zotero - reference manager tool (very very cool) - to help you collect, manage, and cite your research sources
not an extension, just a tech demo, but a first-person shooter demo just using JS and the
<canvas> tag: http://www.abrahamjoffe.com.au/ben/canvascape/