This is a nice addition, but I honestly could do without new Firebug features for a while. It already has 98% of the features I need on a daily basis, and they've all been getting progressively worse in performance for at least the last 6 months. For example, I fired up an older VM the other day that happened to have FF3 and Firebug 1.2 and was amazed that it was so blazing fast compared to what I'm used to with FF3.5 and Firebug 1.4+
I like that it "[lists the] event targets that would be used as DOMEvent.currentTarget when event bubbles". Understanding event bubbling is a bit hard when you first do it. This will make it a lot easier.
This add-on will show you what JavaScript gets to run on a web page, even if it is obfuscated and generated on the fly. Simply open JavaScript Deobfuscator from the Tools menu and watch the scripts being compiled/executed.