Some solution force flash on youtube Rated 4 out of 5 stars

1) FlashtoHTML5:
To play videos above 360p in firefox make sure the following is set to true

For Firefox go to about:config and ensure all of the following are set to True:

media.mediasource.enabled
media.mediasource.mp4.enabled
media.mediasource.webm.enabled
media.fragmented-mp4.enabled
media.fragmented-mp4.ffmpeg.enabled
media.webm.enabled
media.fragmented-mp4.enabled

Html5 lag fix
layers.acceleration.force-enabled set it to true.

If u want to play any video in flash instead of html5 change video url from „watch?v=” to „v/”

example

https://www.youtube.com/watch?v=FqFNKG-z1AA

to
(copy past in tab, direct link not work!!!)
https://www.youtube.com/v/FqFNKG-z1AA

source: " https://productforums.google.com/forum/#!topic/youtube/CUaTWvKhAuE "

2)
Install YouTube Flash Player plugin and install greasemonkey scripts
from:
https://github.com/juneyourtech/GM_YT/blob/master/uTube_force_Flash.user.js

Enjoy )

This review is for a previous version of the add-on (2.0.0).