Firebug Extensions
YSlow measures your site’s speed against Yahoo’s rules for high-performance websites
Firecookie shows all the cookies being accessed by the current web page
FirePHP brings PHP debugging into Firebug using special “X-FirePHP-Data” headers that are invisible in the browser. FirePHP requires a PHP library to send the debugging messages
Pixel Perfect helps you create designs that are just right. You can overlap a partially transparent mockup above your actual web page. Then use Firebug’s standard CSS controls to find just the right settings to make the design pixel-perfect.