Developer Toolbar 4 Firefox named Firebug

download such a advance tool for web developer for debugging in Firefox named Firebug,

Firebug is free and open source.

you can read it's features there and also show screen shot.

• Just the way you like it
• Inspect and edit HTML
• Tweak CSS to perfection
• Visualize CSS metrics
• Monitor network activity
• Debug and profile JavaScript
• Quickly find errors
• Explore the DOM
• Execute JavaScript on the fly
• Logging for JavaScript
  
  

click here to open Firebug site.

click here to find another tool like this