Edit again: the auto focus-switching thing seems to be fixed :)
Edit: Okay I found that Firefox Multi-Account Containers extension and that's pretty much exactly what I was looking for so... :D
Fantastic. I would love a 'generic' implementation of this for any arbitrary site! Though I don't know what specifically goes into containing any single website in terms of maintenance when that website updates or uses unexpected content networks/resources and that sort of thing...
My biggest complaint is that it should not automatically switch focus to the FB page when I open FB in a tab. If not for that, 5 stars.