By default, Firefox allows sites to automatically play videos. I find this behavior annoying and Firefox does not have a simple on/off switch to prevent this. However, it's still easy to resolve this by using the following steps:
1. Start by entering "about:config" (without the quotes) into the address bar. You may receive a scary warning about voiding the warranty and other potential problems. If you get the warning, click "I accept the risk!" to reach the configuration screen.
2. In the search bar, begin typing "media.autoplay.enabled" (without the quotes). Once it comes up, you'll notice that by default the value will be set to true. Change the value to false by double-clicking the line.
That's all there is to it. Note that you may have to restart Firefox for the change to take.