Firebug is the most popular and powerful web development tool.
- Inspect HTML and modify style and layout in real-time
- Accurately analyze network usage and performance
- Extend Firebug and add features to make Firebug even more powerful
- Get the information you need to get it done with Firebug.
See some highlights:
Firebug start button in Firefox toolbar
Inspector API for Firebug extensions
Large Command line has history