From the Mozilla blog:
This extension helps you control more of your web activity from Facebook by isolating your identity into a separate container. This makes it harder for Facebook to track your activity on other websites via third-party cookies.
When you install this extension it will delete your Facebook cookies and log you out of Facebook. The next time you visit Facebook it will open in a new blue-colored browser tab (aka “container tab”). In that tab you can login to Facebook and use it like you normally would. If you click on a non-Facebook link or navigate to a non-Facebook website in the URL bar, these pages will load outside of the container.
Sounds like a great alternative for those who can’t or won’t get rid of their Facebook. This won’t protect you on mobile, though, where I assume most of Facebook’s traffic comes from.