fiveseven_'s comments

I believe it is enabled by default now, and the value in about:config is `media.block-autoplay-until-in-foreground`

Isn't that button for acknowledging the fact that the site will use cookies?

Flexbox Game doesn't support Firefox 53 yet, but I can vouch for Flexbox Froggy as well as Flexbox Defense http://www.flexboxdefense.com/

here's a non-flexbox solution, but the parent element has to have a set height:

  .parent {
    position: relative;
  }

  .child {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
  }

Material Design Lite is released and maintained officially by Google, whereas Materialize is by a third party

semicolon doesn't work on Firefox Developer Edition 37.0a2