How do I pin a website to the dock in OS X?
Okay I just found a way.

You have to drag the website from Firefox left-hand side of the URL bar to your desktop. It will make a new file.

Move it into a permanent location, like Documents. Then drag that to the RIGHT-side of the dock. It will pin. Oddly it won't let you arrange it in the order you want to. It must stay on the right.

But the only catch is you cannot delete the file in Documents. If you do, it'll just show a question mark when you click the pinned icon.