If you do not want all those videos auto-playing in Mozilla Firefox then you can just turn them off.
From a tab in Firefox enter the following:
Search for the option “media.autoplay.enabled” the value should be true click on it to change it to false. You can now close the tab and you are done.
NOTE (10/10/2017): It would appear the the newest revision of FireFox broke this when they added a new value. You will also need to change. “media.autoplay.default” this is by default 0 change it to 1.