The most popular and powerful:
- 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
Firebug is a free source plus it’s an extension to Mozilla browser which can be easily downloaded and used.
Get the latest version from Mozilla addons