Chrome has had it for a years, and it's a killer feature for many developers. It's very very useful to have multiple browser windows open, each logged into the same site as a different user. A lot of people do this by opening multiple browsers (FF, Chrome, Edge, Safari, etc) but that has its limits and it just adds another variable my poor brain would prefer not to handle.
Also very useful for home/work separation. One browser account for work and one for home. And also maybe one for porn. So that when you're screensharing in a meeting and you type a URL into the browser, you don't get autocomplete suggestions for your favorite porn sites popping up. Happened to an old (married) boss of mine once while displaying his screen on the projector... typed "a" into the URL field and the browser helpfully suggested he navigate to AdultFriendFinder. Right in front of some clients. :)
Firefox's "container tabs" implementation may be slightly confusing. Chrome's implementation is dead simple. One identity per window, and the identity name is always displayed in the upper right.
With FF's container tabs, I'll have one identity per tab, and I can see they're color coded, but that means I'll have to mentally map colors to identities. It's more flexible than Chrome's implementation but there's more cognitive overhead involved.
Also, what's up with the name "container tabs?" That tells me nothing about what they do. All tabs... contain things. I think they need to rename it to "identity tabs" or something. How on Earth would anybody ever guess that "container tabs" is related to identities and data sharing?
We'll see how it plays out though. I'm excited to try it and I am continually grateful for Mozilla's efforts. In fact this reminds me I haven't donated to them in a while....