Kubernetes

Serious answer:

> For almost a decade, many of you have relied on our Multi-Account Containers extension to keep work, personal, and privacy-sensitive browsing separate without needing multiple browsers or profiles.

> We’ve heard your feedback and understood the value you find in that separation. Now, we’re bringing the power of the Multi-Account Containers extension directly into the heart of Firefox for all to benefit from.

I'm confused too, maybe it's preinstalled, but disabled?

I remember seeing this Fb container symbol in a tab from time to time in my private machine, but I think I did enable something in advance.

Or it's that I just see it extremely rarely due to uBlock origin.