FireBug is an addon for FireFox 2.x and 3.x. FireBug allows you to easily navigate HTML by showing the HTML in the bottom of your browser (when turned on) and highlighting where that HTML appears in the page above. FireBug also displays CSS hierarchy for any HTML tag you select. It displays the most recently applied styles on top, the oldest applied styles on the bottom, and crosses out (but leaves viewable) old styles that are replaced by newer styles for that HTML element. Very handy in finding where your styles have gone wrong (or in HTML and CSS you did not write).

The FireBug addon for FireFox is available here:
https://addons.mozilla.org/en-US/firefox/addon/1843

The FireBug homepage is here:
http://getfirebug.com/