Today I discovered two fantastic extensions to Firebug.
First, I discovered that Steve Souders (creator of YSlow, another excellent Firebug extension) wrote LiveCoder, which allows you to edit and save your code on the fly. It has been on my wish/todo list for a while now, as I use Firebug pretty heavily for testing changes on the fly (especially CSS). Pretty handy when you're making pixel-perfect (emperfect?) layouts or trying to decypher a bug. Seems natural that you should be able to save your changes once you've made them, no? WYSIWIG, people, it's obvious.
I hesitated about posting twice on the same day because... well, I want to watch Lost. But I just came across Firecookie and I really think you people should check both out. Firecookie allows you to tinker with your cookies (not that you couldn't with other extensions) within Firebug.