Mullvad browser comes with Mullvad VPN extension preinstalled and it is safe to disable that. Althought MB recommends against installing extensions, I am wondering if Proton VPN browser extension can be installed without affecting the fingerprint.
No, just don’t.
Also, Privacy Respecting Web Browsers for PC and Mac - Privacy Guides.
Mullvad Browser comes with the uBlock Origin and NoScript browser extensions pre-installed. While we typically discourage adding additional browser extensions, these extensions that come pre-installed with the browser should not be removed or configured outside their default values, because doing so would noticeably make your browser fingerprint distinct from other Mullvad Browser users.
Just put the ProtonVPN on either the OS level or on the router itself to cover the whole network