Paint Flashing is now in the official Release channel starting in Firefox 11. I posted a new add-on that let’s you enable Paint Flashing with one click: https://addons.mozilla.org/en-US/firefox/addon/toggle-paint-flashing/
This tool is useful for quickly inspecting how Firefox renders your web pages as it visually indicates how much screen area is being painted after the page layout is computed.The source code for the add-on is also posted.
One of the Gecko Layout & Rendering team’s main responsibilities is the continuing development of CSS in Firefox. I recently modified the CSS style system to allow nested rule parsing. This bug fix taught me a lot about the CSS parser and how styles cascade through the rest of the Layout engine. It took me a little while to set up a dev environment, understand the bug, write tests, get the code written, reviewed, and checked in. I now have a much better understanding of what it takes to move code through the Mozilla source trunk.
I suspect that there are other programmers out there currently lurking around the Mozilla community, intimidated by the scale of the source tree, and wondering where to start hacking on Firefox. I highly recommend starting with the Gecko Overview started by L.David Baron, Mozilla Principal Software Engineer, to help beginners understand the browser engine. Thanks, David!