

Ok we’re talking at cross purposes.
Yes, mullvad has a noble repo, it doesnt have a xia repo (which is the mint equivalent version name)
Attempting to add the mullvad repo using the old ubuntu instructions failed because noble =/= xia
Yes you can work around that but its not beginner level.
Op asked if Flatpaks was the answer whivh imho it isnt, the best answer is downloading the .deb
Depends on your settings. Firefox has a setting for delete cookies on exit. It is not on by default though