Between Brave, Mullvad and Librewolf, you really can't go wrong. They all offer great privacy features and ship with adblockers out of the box (the latter 2 ship with uBlock). It's worth mentioning however that the chromium project is going to transition to MV3, which will cripple the functionality of a load of addons, including adblockers (although Brave's built in one will be unaffected).
As for Firefox, I recommend you steer away from it and choose one of the aforementioned forks of it instead because they will give you better results in terms of privacy. Alternatively, you could use the Arkenfox user.js, however it's no longer as well maintained due to being made redundant by the plethora of privacy browsers out there already.
I'm also going to go ahead and give an honourable mention to the firefox css community who have made some insanely impressive themes for Gecko based browsers. You can view some of them here: https://firefoxcss-store.github.io/
They're one of the main reasons I'm using a Gecko browser :)
If you're satisfied with mullvad, i would stick to it. The main problem it runs into is that it doesn't support port forwarding which is an issue if you use torrents a lot. That's the main advantage that proton reserves.
In my stomach