Extremely Valuable Rated 5 out of 5 stars
This has to be my favorite add-on. Honestly, I like it more than Firefox itself (sorry Mozilla). Firebug is a very detailed add-on. It has everything you need... well, almost. It could definitely use a one-line statement evaluator, similar to what Execute JS does (except in a single line on the status bar), but you can easily supplement Firebug with Execute JS until the developer figures out how great that would be.
Every update gets better, and I don't think there's another add-on in all of the add-ons arsenal that receives as much detail oriented attention as Firebug. One of the best features is the Inspect Elements Overlay, that is perfectly integrated into the HTML console. You can easily find an item in the code by clicking the inspect element button, and then hovering over then element.
As you become more an more familiar with Firebug, you begin to see that this add-on is far more powerful than you first imagine. Edit live HTML. Edit CSS on the sideline like in Dreamweaver or as a stylesheet. View the DOM as an actual tree. Pause and break script or watch expressions to see their output. Measure network data. Find script as it fires. Toggle viewing of Whitespace, comments, full text, and more. The list goes on and on.
I couldn't do without this add-on. Supplement it with Execute JS and Console² (Console Squared if that didn't come out) and the Web Developer Toolbar, and you'll have everything you need to develop, test, and modify with ease.