This article aims at listing ways to debug Firebug and its extensions.
FBTrace is a Firefox add-on to use with Firebug. You're offered an interface that lists logs while Firebug is running.
 Browser Debugger
 dump() and the System Console
At some point good old dump() into the system console can be also an effective strategy how to hunt a bug.
Chromebug is not maintained anymore at this moment. The Browser Debugger is a replacement for this.
 HTML/CSS debugging
 DOM Inspector
DOM Inspector is a Firefox add-on which is able to change CSS/HTML of chrome documents. Rather cludgy UI. Start by "File -> Inspect Chrome Document -> Firebug".
 Element Inspector
Element Inspector is an add-on to use with DOM Inspector. Just press Shift and right-click somewhere to display the matching element in DOM Inspector.
If DOM Inspector is not installed, it opens it in Firebug.