If I may ask, what’s the significance of using a 3rd browser to stay logged into sites? I know there’s a convenience factor here, but I’m pretty sure arkenfox’s hardening script makes Firefox dump all cache and cookies after you close the browser. So when you relaunch it you won’t be signed in on any accounts. I’m unsure about Brave’s stock settings, but it probably does something similar.
Mullvad is for fingerprinting.
Brave and Firefox do offer some fingerprinting, but not enough to make a real difference.
If you don’t care about anonymity, then you don’t need Tor.
If you don’t care about fingerprinting (blending with the crowd) then you don’t need Mullvad.