How do these icons work: 🌍🌎🌏✅️?

I can see these characters as colored icons:

🌍🌎🌏✅️

It only works in Firefox for me. If you can't see the characters in color, it looks like this on my system (it's probably font-dependent):

I can even see them in firebug and tab titles:

And there's full file of them. I also did this document.title animation with them, which demonstrates that they work even outside HTML.

What is that?