It is 100% free and open source. And one thing to note, do not mistake Firebug Lite with Firebug. It is not a substitute for Firebug or Chrome Developer Tools. It is a tool to be used in conjunction with these tools. Firebug Lite provides the rich visual representation developers are used to seeing in Firebug when it comes to HTML elements, DOM elements, and Box Model shading. It provides some cool features like inspecting HTML elemements with your mouse as well as live editing CSS properties.
Firebug Lite is better than no Firebug at all in Chrome.