That’s the Mullvad Browser. I know that it’s available on Desktop, in fact, it’s only available on Desktop. OP is referring to the Mull browser, which has nothing to do with Mullvad VPN or Mullvad Browser. Mull is a fork of Fennec (Firefox for Android) created by DivestOS. It’s basically Firefox for Android with arkenfox user.js and some other neat privacy tweaks.