Dumb question: Does the implementation have anything to do with containers? E.g., Docker? Or is this just overloading an existing industry term?
It's not really overloaded here. 'Container' means a contained environment that can see itself, and cannot see other contained environments on the same machine/network. It has the same basic meaning for both Docker and Firefox.
Docker was released in 2013, so if you search for "software container" or "software virtual container", or "chroot container" on Google filtered to before 01-01-2012, you will find plenty of examples of it from the past.
LXC - Linux Containers – was released in 2008... Virtuozzo Containers since at least 2000.
Here's something called Aurora for containerizing CORBA services in 1998: https://link.springer.com/chapter/10.1007/BFb0054506?no-acce...