How do I block Facebook's website from Firefox? I use Apple OS X.

How do I block Facebook? I use Apple OS X. I canceled my Facebook account, but there are sometimes advertising links on other sites where you can't tell it's a Facebook site until after clicking on the link. The problem with Facebook is if you cancel your account but then visit again (accidentally or otherwise), it reactivates your old canceled account--unless they've changed their privacy settings again to stop this. How do I set up Firefox so that it blocks Facebook, or at least warns me that I'm about to go go Facebook? Thx.