I installed Mullvad browser and firefox snap in an Ubuntu VM and there was no warning about user namespaces. When i went to about:support in the sandbox field it showed me that user namespaces is not disabled.
But when i installed Librewolf the warning was there and user namespaces was disabled.
I’m not the one to answer your question, but I was under the impression that Snap packages didn’t interfere with AppArmor.
I’ve noticed that some browsers produce the security warning when installed as a flatpak, but not through APT. Others produce the warning when installed through APT, but not Flatpak. I’ve done a lot of reading on that security warning, but never really could figure out a solution.