Edge really needs end-to-end encryption for sync and better privacy policy

ragingrei IMHO this thread remaining open is misleading. The use of "primary password", or former "master password" for example in Firefox is quite different than what is implied by your statement. It is utilized for local encryption.

Typically this is not be required in a user's environment in Windows as apps and users can benefit from other means to protect their local data with their logon password. I won't go into details.

Of course data are encrypted by all browsers when synched with a backend service in a manner that in theory is not reversible by the service owners, as it requires the original account password of the user account on the service. The latter is available to the local browser but typically not to the service itself as it should in principle only store the hashed password.

Of course there are several techniques that allow secondary keys (backup or recovery keys) to unlock encrypted data with user's password, however I want to believe that none of the big ones employs such a technique... I hope I won't prove wrong, but in any case this has nothing to do with the master/primary password thing.

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