When I go to a page that has a video, maybe I'll play it, maybe I won't. If I'm already playing another video, I definitely won't, I can't listen to two videos at once.
But HTML 5 lets videos decide for me.
To disable this in Firefox:
Type about:config in the browser's address bar and hit enter.
Confirm that you will be careful if the notification comes up.
Find the "media.autoplay.enabled"
Double-click on it.