Last week I ran into a peculiar problem at work where one of the features on one of our AJAX product was not working as it should be .
How does one debug this sort of problem anyway? Doing a view->source was making me brain dead. The amount of obfuscation in the code is amazing. After a little poking around, I stumbled across a firefox extension called firebug (which I had already installed. Doh!) which showed me that one of the GET requests to a outside property was failing with a 404.
Once I nailed the problem, it was as simple as getting them to fix the 404 and the feature started working again. Firebug is very impressive and handy.