If you're also having problems with fullscreen youtube 1080p playback being choppy, the following helped me:
Before proceeding, make sure you have the latest HD Graphics driver for the Z3700 bay-trail from https://downloadcenter.intel.com/search?keyword=Z3700 . At the time of writing this it's 15.33.42.4358 off https://downloadcenter.intel.com/download/25698/Intel-Graphics-Driver-for-Windows-7-8-1-10-15-33- .
Now, in Firefox's about:config, filter for webm and enable the following booleans:
media.mediasource.webm.audio.enabled
media.mediasource.webm.enabled
media.webm.intel_decoder.enabled
That should do it. Restart firefox, and good luck 
