Joe Hewitt has announced Firebug 1.0 final, after sneaking support for viewing computed styles into beta 9.  Since I started using Firebug in it’s 0.3 days, it has completely changed my approach (and attitude) to front-end design and programming work in HTML, CSS and JavaScript.  As a Firefox extension, it’s immense functionality and beautiful simplicity continue to amaze me.
As Joe points out, there are tens or hundreds of thousands people still using the previous stable 0.4.1 release, which was downloaded 840,000 times from the official Firefox Add-ons site alone!  They are going to get a very nice surprise when the automatic update comes through.