I've tried Firebug pretty early when it was first released and wasn't too impressed. I was too obtuse to find my way around it, and Gmail, IIRC, was givin way too many errors at the Firebug indicator, my machine was just grinding to a halt. Or maybe I just wasn't doing too many web dev stuff back then (was doing Eclipse plugin) to be really in need of installing it.
For the interested, I'm now a Firebug fan. Its been helping me greatly in debugging webpages. Yea.. debugging web pages, however pointy-haired that may sound.
E.g. this screenshot here showed how I could easily tell why the grey area (a "pre" tag) was blank - by just doing a mouse over. (see more...)
Awesome! Do lemme know if there's an equivalent (hopefully free) IE tool for this. It'll really handy to be able to inspect the current (js modified, not 'view source') DOM representation in IE.
Note: after installing Firebug, click on the green tick (or red cross) at the bottom right corner to bring up the Firebug panel.