I'm a little confused here, I've been using the containers feature already for years now on Firefox. And I have no add-on installed for it, it just works.

So what is it that they're adding?

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.

You could have containers but the software itself had no way to manage them. The extension provided the de facto default way to manage them. I think now they are baking the functionality in. I wish they would actually flesh out split tabs. It is so barebones it pissed me off enough to sort of stop using Firefox on a few boxes altogether after decades of use.The browser seems to be the last thing on this company's mind.