Yes, I test it.

In general is not a big deal than any other "privacy" browser.

Mullvad is a great browser, especially as a default browser (for opening links) as it will clear all cookies / local storage every time you close it down. So it's ideal for random browsing.

Every time you open, it is like a fresh browser. It also brings a whole load of extra protections from the Tor project (such as reduced fingerprinting).

For sites you visit a lot, and want to stay signed in (like stacker news) then you should use a regular browser. Also some sites won't work, due to all the tracking protections.

They're a step up from Proton in that they haven't been known to provide user data, even know Proton never really gave user data, only a users recovery email and suggests you don't use one if you're a whistleblower etc. Mullvad is nice cuz you pay for it with gift cards, even some physical stores sell them,which is pretty anon. You can pay proton in cash through the mail, which is also pretty cool. It's a toss-up between them two but what wins for me is Proton being in Switzerland, that alone is fortification.

I like their VPN service, but I've found Firefox to be the best browser. Still, Mullvad is more secure. I need to use Firefox because sometimes sites break when using Mullvad, which is why I prefer it. My goal has always been to use a non Chromium based browser and Firefox is my go to choice.