As people have said containers is amazing. But in the scenario where you really need 2 completely separate browsers and don’t want to use different operating system accounts (I’ve done this in the past).This extension has worked well for me https://addons.mozilla.org/en-US/firefox/addon/profile-switcher/ It’s not ideal that it’s an extension but it does do its job

run in a VM/container?

Just gotta get someone to hack their system. Then it’ll be easy to prove damages