While the debugger is stopped, the Firebug toolbar contains a horizontal view of every function in the call stack. Click a function to move to the file and line where its execution stopped and inspect the local variables in that context.
Error messages have a red button that you can toggle to set a special kind of breakpoint that stops whenever the exact error occurs again. Toggle this button on, then try to reproduce the error. Firebug switches you to the debugger when it happens.
Sometimes, Firebug can save you the trouble of having to reproduce the error. Error messages have an attached stack trace that you can see by expanding the description. The trace exposes not only the names of the functions on the stack, but also the values of each of the arguments that were passed to each function.
The debugger Keyword