FireBug is the most important tool for Web Developers, period.
I will write more on this later but as I develop for an intranet project using Dojo DataGrid 1.2, I was getting errors when loading and making asynchronous requests to the data page. These errors would be another guess and check game for hours and hours. Luckily FireBug for FireFox showed me the requests being made in the console and the data returned by those requests. I was shown the errors my datastore was returning and I was quickly able to mitigate the issues.
THANKS FIREBUG!!
