Greasy Fork is available in English.
Buffers the video without autoplaying and puts it in HD if the option is on. For Firefox, Opera, & Chrome
< YouTube Auto Buffer & Auto HDについてのフィードバック
Yeah, YouTube now defaults to playing videos in HTML5 if your browser supports it, which this script doesn't support yet unless you install the Beta version.
The script has supported HTML5 for quite some time; it's just broken right now. It does work in the beta, though. I'll probably be rolling that out into an official version, soon.
Script does not pause videos and no auto HD 2015/02/18
The options menu works fine, but the Auto-Pause/Auto-HD does not work with Youtube videos :/
Firefox Version: 35.0.1
Edit:
just switched over to the Beta version its working fine . the regular version is broke