Lock firefox profiles
In the past (loooong time ago) I used an extension to password protect my browser, had multiple profiles and for each you could only enter with a password, since that is no longer avaiable we still have the "Master Password" that would be great too if there wasn't the "cancel" option, you see, when the prompt appears you can't click anywhere else, tabs, bookmarks, nothing, so if there was a way to remove the "Cancel" option from the prompt that would be a great measure for security, is there any way to remove it from the code or something?
Why not just make user account password to lock each account from non-account user through Windows 10. Sync isn't a backup tool.
Currently, the Master/Primary password only protects saved logins, it doesn't gate access to browsing (or your bookmarks, history, etc.). In response to your question, there isn't a built-in option to make the dialog non-cancelable. Is there some kind of hack? I don't know of one because even if you were to hide the cancel button, tapping the Esc key will cancel the dialog.