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.

No, it's at about:preferences#containers and has been for years if you turn it on in about:config (privacy.userContext.enabled and privacy.userContext.ui.enabled).