I often talk about the benefits of using Firebug for Firefox in the MSDN Design and Customization Forums. Rather than try to type out a long winded article on why it rules, I found a video that should help to convince non believers that it is truly superior to the IE Developers Toolbar:
Firebug 1.0 Beta Screencast by Jesse Newland
Takes a little bit of time to download the video, but its worth it.