Mozilla/Firefox Web developement Extension

http://www.chrispederick.com/work/firefox/webdeveloper/

This is a very handy development extension for Mozilla/Firefox.

Several tools such as validation, forms tools, information viewing (CSS, HTML, speed reports), Image functions, and many more, can be accessed from a tool bar or context menu.

I think my favorite thing is being able to modify the CSS of a website and seeing the changes instantly. Great for troubleshooting other peoples websites.