Rated 5 out of 5 stars
As a web developer, I use this every day with deep gratitude toward the developers.
Firebug is really easy to use. In the years I've been using Firebug, it's revolutionized the way I develop web pages, and I've never once had to consult the manual.
Before this add-on came into my life, fine-tuning the appearance of a page was a long process: where I changed the file, uploaded the new file to the web server, refresh the page and hope for the best. Inevitably, it would be repeated dozens of times. Now, I just make the change in Firebug and immediately see the results. Making pixel-perfect renderings of Photoshop files is an order of magnitude easier with Firebug.