Blocks videos successfully, but exceptions list does not work. HTML standards bodies need to come up with a real option for disabling autoplay instead of these half-baked, buggy solutions.

recently vimeo videos don't play with this addon enabled, even with selecting not to disable auto-play and preloading

per-URL config is wonderful (e.g.: disable-autoplay-everywhere-except-youtube)

EDIT: it stopped working completely for some reason

All this does is toggle the setting for autoplay which can be done manually. No way to disable per site. Useless.

Buggy. Doesn't let me refresh https://www.youtube.com/feed/subscriptions

Twitch being the worst affected. Video doesn't load at all when it's not whitelisted.

This add-on certainly stops content from autoplaying as intended, but currently will not save your preferences permanently.

For example, one has to disable the add-on each time you want to watch Netflix, no matter whether you stay on the same tab or ask to save your settings. This is on Firefox 50.1 (up to date as of this writing). Unsure what the bug is, but I'm not alone in experiencing this issue.