My favorite Firefox addon would have to be the
Codetech addon. It allows you to quickly edit webpages in WYSIWYG (What You See Is What You Get) Mode, and it really saves time when you can do that right inside a browser.
Another addon I like is the
Greasemonkey addon. This addon allows you to create your own userscript and edit the way the websites load. You can browse through a website, click on a tool on the Greasemonkey toolbar, and delete/modify a setting on a website, and the script creates a custom userscript for you and loads it the next time you go to that certain website.