Here's a tip that might come in handy if you use Firebug a lot.
I had major trouble with it this week. The script debugger stopped working in several nasty ways.
The problem turned out to be a corrupt Firefox user profile. Creating a new one restored correct behavior.
I'd never have thought of this on my own (the answer came from http://groups.google.com/group/firebug) and it was a big relief as I depend pretty heavily on Firebug for my work in JS.