We're waiting on this implementation over at http://lookback.io. We're currently recording four input sources (camera, screen, microphone and taps/touches) and need to sync them all with a single progress bar. We're able to do this in Safari and Chrome but currently can't support the Firefox browser due to the missing MediaController.
Given that our product is OS X heavy, we could live without it, but as we love Firefox and would love to see it grow on OS X, we want to support it, too.
Looks like Chrome will unship MediaController for now