When I develop a web application I cannot do it without the Firefox add-on Firebug. Firebug is a add-on with great features like editing and debugging HTML, CSS and Javascript. Also a good monitoring tool to see which files are loaded for page, their sizes and download time. And much, much more.
FireCookie is an add-on to the Firebug add-on. With FireCookie we can easily see and change cookie values. On my latest project this would have been very helpful if I had known about this add-on earlier.
