You should have a "guest" Logon User Account in Windows 10! https://support.microsoft.com/en-us/help/4026923/windows-10-create-a-local-user-or-administrator-account The Master Password feature in Firefox doesn't keep other people who have access to your computer from using Firefox. That feature is intended to prevent other people from viewing your Firefox login data (usernames / passwords) and from using that data for logging into any websites that you have saved password for. Once per session the Master Password would need to be entered.