[c0llision]

You would need to do that for every domain firefox talks too, which will quickly get annoying, as stated in the original comment:

Safari and Chrome want to talk to all kinds of things on TCP/80 and 443, so you pretty quickly say they're allowed to make any 80 or 443 connection they want without further pestering you

[m463]

You can set it up to allow/block a site or domain forever.

For example, block graph.facebook.com forever.

You can also look in the little snitch network monitor and block sites after the fact for the future.

It's a lot less tedious if you know the keyboard commands:

alt-return denies the connection, and cmd-return allows it.