By far the coolest add-on for firefox I’ve seen for developers since firebug. It’s also available for Flock and Seamonkey.
This thing gives you all sorts of information about the page that you’re viewing. Some of the toys available:
- Line Guides
- Ruler
- Display Div Order
- Display Element Information
- Resize Window
- Dom Inspector
- Validation tools for CSS, Feed, HTML, 508
- Outlining of Frames, Headings, Links, Tables, Block Level Elements, Positioned Elements
- Display Image File Sizes, dimensions, paths
- Disable CSS, Javascript
- Inline editing of Html and CSS
There are a ton of other features… well worth the donation.