via https://developer.mozilla.org/en-US/docs/persona/branding
If they recognise it, then they'll know. If they don't, then they don't need to.
1. http://sloblog.io/login has a nice, explanatory landing page.
2. https://www.voo.st/ has a small string of explanatory text at the point where a user chooses between Facebook or Persona auth.
3. http://crossword.thetimes.co.uk/ has a simple, unbranded "log in" button that just opens the popup.