Still developing without a debugger? Shame on you.
And thank goodness for Firebug! For old skool developers who learned their craft before the Internet was around, debuggers were de rigeur, and not just because they helped to track down nasty problems. A good use for debuggers in the old days was to step through every line of code, watching the state of every variable as it changed during the course of execution.
Today, the benefits of having a debugger are the same: the ability to isolate problems; the ability to see what's happening and understand the nuances of code changes; and the ability to profile critical sections of code to improve benchmark timings.