You can set Master Password on your Firefox, See: * https://support.mozilla.org/en-US/kb/Protecting%20stored%20passwords%20using%20a%20master%20password * http://kb.mozillazine.org/Master_password

The master password is only used to encrypt passwords saved in the password manager and doesn't prevent from opening Firefox.<br /> You can't prevent that.<br /> There are extensions that allow to open a profile only after a password has been entered, but extensions can easily be bypassed if you know how to do that e.g. by holding down the Shift key.<br /> See also: * https://support.mozilla.org/kb/Parental+controls * http://kb.mozillazine.org/Parental_controls