I don't know why mullvad and the tor org wouldn't just do it....

Lol, down vote because holy crap do you know what mull is and why that may be the best option?

Mull is kinda sorta Tor Browser Android, but without routing over Tor network and upstream Firefox. It enables many features upstreamed by the Tor Uplift project through @arkenfox and @pyllyukko's user.js projects.

And considering mullvad doesn't have a mobile app yet..

https://mullvad.net/en/browser