Rated 5 out of 5 stars
One of the first tools I install when reinstalling the OS or Firefox. Apart from the obvious and most used web dev functionalities, I have other uses for Firebug.
Can help in situations a "smart" dev makes part of a site work only in IE. Just open up Firebug, make a few edits over here and there, and you are ready to go. I find it annoying to have to go to another browser in order to just one task.