Firefox 3 had over 8 million downloads the day it was released. Firefox has helped us all in web design and development.
You can get Firefox 3 from the regular Mozilla download page. In order for Firebug to work with Firefox 3, you will need the latest Beta release, Firebug 1.2, available only from the Firebug Releases page.
The ability to view and manipulate the entire DOM of a web page after it is rendered is amazingly helpful and helps in smart development.
I just love the way that this works, it becomes a part of the Firefox itself and has started a new trend of browser specific development tools.