Although Firebug is already packed with features, there are always rooms for extras. Since the creation of Firebug, plugins are being developed to extend the power of Firebug even further. Here is a list of useful plugins for Firebug.
YSlow developed by Yahoo! is probably one of most famous Firebug extension out there. It analyzes web page performance and tells you why performance is slow. It gives you information on all the requests, load time, sizes and even give you tips to improve your site speed.
Instead of using other Firefox extensions to view cookies. This Firebug plugin allows developers like you to keep using the firebug interface to debug with cookies. FireCookie gives you the ability to view and manage cookies. It also has a console where you can see when cookies are created, changed or deleted.
For those PHP developers out there, FirePHP is enable you to print to the Firebug Console using simple PHP function call. The main benifit is that all the debugging data are hidden from normal user and you can review in a nicely format within Firebug console.