Go to:
about:config
media.mediasource.enabled
make it true
Go to this website
Install “Magic Actions” addon in Firefox. Restart Firefox, go to addons select magic actions options and choose the options according to your taste.
You can choose HTML5 videos, loading videos while you are watching, 480 or HD quality.
Selecting 480 instead of HD quality will save bandwidth (DATA). Which in turn could save monies on your bill.