An extension that has no user prompting or even status indicator, and that will permit the user to browse the web without a VPN connection or warning by default.

It appears that the process is to 1) open Mullvad Browser 2) (externally) open Mullvad VPN and connect to it 3) click on the Mullvad Browser Extension icon and connect it to the Mullvad proxy. Only after this will the proxy be used and the connection secured.

Contrast this with Tor Browser's process of 1) open Tor Browser. It will only work after it automatically connects to Tor and secures the connection. Do you see the significant difference?

Mullvad wants this browser to use usable even by people who don't use their VPN. Tor Browser is never intended to be used outside the Tor network.