Web Developer Extension

Web Developer Extensions at work
Web Developer Extension at work

If you do any website development, then Chris Pererick’s Web Developer Extension can save you a huge amount of time. There is no end to what you can find out about your website with it: highlight selected HTML elements with their names in the document, display the HTML and CSS styles being used, get information about element-nesting, call the various validator services for HTML, CSS etc., disable cookies, java, javascript, and much, much more. Available for Firefox, Flock, Mozilla and Seamonkey on any platform (e.g. Windows, Linux, OS X) these browsers run on.

I wish I had discovered this long ago, it saves so much time debugging errors in my code.