Update firefox.html with new instructions
Update firefox.html with new instructions

As of April 13th the removed steps are no longer necessary for webvr support in firefox nightly. I'm unsure if the 3rd party sources for oculus are still necessary as I don't have an oculus to test with. 

For reference the tweet about this change:
https://twitter.com/kearwoodgilbert/status/852776579750952961
WebVRRocks/webvrrocks,WebVRRocks/webvrrocks