I almost dismissed this as an April Fools day joke. Glad I gave it a click through. Firebug Lite.
…but what happens when you need to test your pages in Internet Explorer, Opera, and Safari?
To take it a step further, there is also a bookmarklet you can use to make it very easy to invoke. It is actually easier/quicker to enable in other browsers than FireFox. In addition, you don’t get random sites warning you about a potential slow down.
Highly recommend you check it out.