The implementation of Media Source Extensions (MSE) to support YouTube videos running in HTML5 has begun. We have enabled MSE in Firefox Nightly for YouTube on Windows and Mac, and over the past couple of weeks Mozilla engineers have been working hard on addressing the top bugs. We are looking to get this into the hands of our users as quickly as possible and in order to do that successfully we would like your help.
If you would like to participate, please install the latest Nightly, go to youtube.com, run videos and tell us about your experience. Note that this also covers You Tube videos that are embedded in other sites, not just videos on youtube.com.
Our testing etherpad has all the information to get started, including a template to file bugs if you find them: https://etherpad.mozilla.org/YouTubeTesting
Please feel free to ask questions in #qa on IRC. We look forward to your participation!