Firebug is a plug-in for Firefox that provides a bunch of functionality to help web developers work with client-side technologies. This has to be one of the best tools that I've ever used for web development, and I'm hoping this is the first of a few posts about the tool. There's so much there to talk about that I'm just going to start going from left to right on the Firebug menu.
You can install Firebug from here.
After installing, you'll see an icon in the corner of Firefox:
For instance, I can type in the window "document.title":
There's all sorts of stuff you can do here, execute methods, print cookies, print variable and DOM element values.
This is such a handy tool. If you don't have it, get it, you need it.