Web Developer's Best Friend Rated 5 out of 5 stars

Firebug will tell you exactly what code is used to make what object on a website. The highlighting, side CSS/Layout/DOM display, and instant editing let you find any issue and attempt to fix it live before saving the fix in the source code. This extension is going to be a favorite with Website Developers for years to come.

This review is for a previous version of the add-on (1.4.3).