Like most things I've tried -- I installed earlier versions without reading any manuals and found earlier versions to be very intuitive and helpful without any training.
The latest release, FireBug 1.0, contains many new features that are more subtle. The best way to learn about the power of FireBug 1.0 is to view this video presentation from the author Joe Hewitt. I experienced many of those - "I didn't know it could do that" moments.
So if you're not already using Firefox as your web-browser...switch to it! And after you've done that -- install this plug-in.