This is an open issue on the Mullvad Browser GitHub.
One person in the thread who's using Pop OS posted this:
xdg-settings set default-web-browser start-mullvad-browser.desktop
However they do report the clicked URL not being passed to the browser, so it doesn't seem to be that good of a solution.
This reply worked for someone on Arch though.
I'm having the same issue, so I will try to fix it later and get back to this post if any of these methods work.
EDIT: I tried both methods. The Arch one did nothing, while the xdg-settings one did set Mullvad Browser as my default, but when I click a link the browser will open on the homepage anyways. So yeah, seems like we gotta wait for this to get fixed by the Mullvad devs.
I had tried that but Network Manager couldn't read the config or something. Anyways I fixed the issue by enabling quantum tunnel, but thank you anyways for replying!
Thank you, selecting Wireguard only and activating quantum tunnel fixed it!
VPN not working
I'm running Pop OS 22.04, everything updated. I've been trying to use Mullvad VPN, which I've installed using the latest .deb on their site, but as soon as I connect to a server I seem to lose internet access. It shouldn't be a problem with my network because the VPN works perfectly on my other devices, even with the same settings. I've also tried with other servers and other versions of the deb. Then I also tried with two other VPNs (Proton and RiseUp), neither of which worked.
At this point it has to be something with how the connection is configured on Linux. I've left everything as default.