Watch Youtube embed movies on third party sites only in HTML5 on Firefox, Chromium/Chrome, Opera. This scripts also repair loading old youtube flash embed movies on Firefox.

1. Install Script manager for your browser https://greasyfork.org/help/installing-user-scripts
2. Install scripts
https://greasyfork.org/scripts/13174-replace-old-flash-player-based-youtube-embeds-by-their-new-html5-counterparts
and
https://greasyfork.org/scripts/3435-youtube-embed-tweak-html5

3. Execution order script, 1st is "Youtube Embed Tweak HTML5" and 2nd "Replace old Flash Player-based YouTube embeds by their new HTML5 counterparts". 

4.In script "Youtube Embed Tweak HTML5" change var videosize to 100%(Original size video width and height, but sometime is larger. This is problem html5)
Change to default size and settings:
// Set Video Size, with a 16:9 preset, Large (1024x576) medium (720x405) or Set size in percentage, for a video size as a percantage of the container size. 
var videosize = '100%';
// or
//var videosize = '50%';
// nochangeurl must be set to no for player settings to work. yes = default url and no = modified url, size is always modified.
var nochangeurl = 'no';
// Show youtube logo, yes or no
var ytlogo = 'yes';
// Show annoations yes or no. 
var annotation = 'yes';
// Show Related videos at end of playback, yes or no.
var related = 'yes';
// Force https option, yes or no
var ssl = 'yes';