If you don't like auto play videos, and I hate em, do this:
Type about:config in the address window in Firefox.
It will give you a bullshit warning. Ignore it and proceed.
Find media.autoplay.enabled
Double click. It disables it and is just as easily reversed.
Seems to work so far.
ADblock might do it too but I quit using it because of Vital and a couple of other sites that won't load if it's on.