The Master Password feature is intended to request your Password the first time that you load a page that needs login information, or when the user attempts to view their saved Passwords. Not intended to keep other people from simply using Firefox.

https://support.mozilla.org/en-US/kb/use-master-password-protect-stored-logins

In all versions prior to 57.0 this was not a problem. The new version (57) changed all that so that I am no longer protected in my browsing history, bookmarks, and cookies.

This feature should not have changed because a new version was released.

If "57" cannot be secured I will uninstall Firefox and be forced to revert to Chrome.

sysmk said

In all versions prior to 57.0 this was not a problem. The new version (57) changed all that so that I am no longer protected in my browsing history, bookmarks, and cookies. This feature should not have changed because a new version was released. If "57" cannot be secured I will uninstall Firefox and be forced to revert to Chrome.

Suggest go to their site. Firefox Support does not do Master Pass Support, they have their own Support Pages and FAQ's and a Support Forum http://help.masterpasswordapp.com/ if their FAQ's do not answer your Questions.

Thank You for Using Firefox Support.

Canceling the MP prompt to continue has always been possible and extensions that protect access to Firefox can easily be bypassed by starting Firefox in Safe Mode (i.e. hold down the Shift key during startup).

You can use a screensaver with password protection that automatically drops in after some idle time. Otherwise make sure to use a guest account for others who have access to this computer and use a (strong) password to protect your own Windows user account.