Odds are it's not one of your favorite sites if the site owner isn't even committed enough to have made a favicon.

Before you smack the reply button to pop up with "But here's an example of my favorite site without a favicon!": I said odds are.

I doubt the number of favorite sites without a favicon is low enough at this point that Mozilla shouldn't spend a lot of time worrying about it.

The example picture has two sites with the same favicon, so the general-case problem probably isn't all that uncommon. Especially if you're the sort of user who does things like checking multiple twitter pages without an RSS feed.