Add support for Mozilla Firefox

Mozilla Firefox is the second or third largest desktop web browser by market share ([Source](http://gs.statcounter.com/browser-market-share/desktop/worldwide)).

Because of this, OCDM should add support for it as well by using Mozilla’s [GeckoMediaPlugins](https://wiki.mozilla.org/GeckoMediaPlugins) API, which is intended to be utilized by CDMs and Codecs.