Absolutely nothing to do with the Master Password feature in Firefox.

Somebody set up Sync in that Profile. No other way that will appear in the Password Manager. Importing data from another browser won't create that chrome:// address. Strictly internal to Firefox / that Profile.

If you open Saved Logins and then Show Passwords, right-click that "Site" and select Copy Password. Then paste that into a text editor. You'll see the kA and the kB string that is used for encrypting your data as it is sent to the Sync server and for decrypting it upon return from the server. Looks like this:

{"version":1,"accountData":{"kA":"b8b02f83306f5f0e038c14b3ed82f585b38a40af1f30c85e85c9d95fcc4446e8","kB":"c7961085aab9766b95eek3cfe85319902fc991958cbdf33259276f645a9ce007"}} Obfuscated strings as it's from my browser Sync account in a test Profile.

I have forgotten exactly what kA and kB are specifically used for, but the combination of those 2 "keys" is what encrypts Sync data when that data is off of the device. In transit to the Sync server and while it's sitting on the Sync server.

Those kA & kB strings are created from the user set Password for that Sync / Firefox Account at the time of Password creation or the last Password change. Without those exact keys, the Sync data is virtually useless; can you imagine trying to "guess" an alpha / numeric string with so many characters? That's why changing the Sync Password deletes the data from the Sync server; with a different Password Firefox wouldn't be able to decrypt the data, so it's just deleted from the server as useless.