Best browser to login multi accounts?

I have a lot of league mates that ask me to log them in for FB/AB when they are offline or keep their aids running. Ive been trying to find ONE browser that will allow me to do this easier. So far it seems I'm only able to use 1 normal window and 1 private browsing tab on each different browser ( firefox, google chrome, opera, explorer and safari). Has anyone had any better luck with this?

Thank you