I think it needs cooperation from the app side. That said, Mullvad has Mullvad browser, which I assume has similar functionality if you're a customer of theirs.
I hope they make this extension available for Firefox Android too. That'd relieve me of using two browsers in my phone, one for logged-in business (excluded from VPN) and the other for general browsing (with VPN).
I thought they gave up on their VPN development.
And YSK: Mozilla VPN uses Mullvad as the backend.