Interactive Javascript console (preferably integrated with Firebug)

I'm looking for a way to have an interactive JIT debugger, preferably integrated with Firebug.

I got the idea from PHPEd, which has an "Immediate" debug tab where you can just type in PHP code and modify objects on the fly. This makes debugging a breeze as you can re-assign variables multiple times, re-execute functions, etc without leaving the program.

Here's what I think would be superb: - set a breakpoint in Firebug - arrive to breakpoint - have an Execute JS tab where one could enter JS code, similar to what I described above

Does anything like this exist already?

TIA.