Does a wonderful job in most cases but does not stop youtube from auto playing in background tabs neither in firefox, chrome or opera. I think Youtube is overriding your setting somehow being the spammy ad monster it is.

One other small issue:
Sometimes videos don't start out in my preferred quality. I set it at 480p for this silly laptop, for instance). So videos wont even start because they're trying to play at 1080 or 720p instead. I noticed the same issue in Opera.