Firebug/JavaScript Debugger

Firebug has a powerful JavaScript debugger that allows you to pause execution any time, and see what any variable looks like at any moment.
Using Firebug to debug Javascript you can:
Sort and organize files into a clean list to help you find any file in easily.
Pause execution at any line. You can set breakpoints, which make the debugger pause the execution at a specific line. You can also set a specific condition to your breakpoints.
Take it one step at a time. When the debugger is paused, you can execution one line at a time
Firebug sometimes offer to automatically break into the debugger when an error occurs .
Stack traces or unstack. When the debugger pauses, It shows you the call stack
Watch expressions. Firebug enables you to type in random JavaScript statement with a value that will be updated whenever you step in the debugger.
Variable tooltips. When the debugger pauses, you can move your mouse over a variable in the function to see a tooltip which reveals the value of that variable.
Go directly to a specific line number, just type in a pound sign then the line number inside the quick search box.