Sorry, Firefox doesn't have a password for limiting use, there is a Master Password feature for preventing other users from using your saved passwords. There is one extension for password protecting a Firefox Profile, but it isn't compatible with MacOSX.

Your best move is to create a "guest" user logon account (whatever Apple calls it) for guest users on your PC, to keep them from having access to your personal data in all your programs. See a Mac user group or Apple support for instructions on how to do that.