I guess why not.

This is an open source, rebranded Firefox and Firefox-like browsers could use some publicity. It promotes privacy and privacy can use some publicity too. Tor too.

Mullvad seems to be honest in the fact that their business model is selling VPNs and it's nice they are saying it's not enough. They are not saying that you might not need one though.

We need a Firefox with good defaults and it seems like this browser is such a thing. I'd prefer these privacy features to be in upstream Firefox but I guess world is not perfect and that Firefox still relies on revenues from Google so can't be as privacy-focused as it should.

My little concern I guess is that this browser will push for their service so it's a bit like an ad for them, at least with its name. But fair enough, and at least the business model seems healthy.

With Mullvad already being a Mozilla partner for their branded VPN, all this actually look good. They seem to be spending their money on worthy stuff.

It's a custom build of Firefox with somewhat sensible, sometimes strict, privacy respecting default settings.

There's also the Arkenfox user.js which you can put on top of vanilla Firefox, aiming for the most privacy and security possible. https://github.com/arkenfox/user.js