I have to strongly disagree with Arc's implementation. It's only a minor step above the least desirable method of having different windows for each profile. At the end of the day, it amounts to little more than a UI tweak on how Safari itself handles it.
If you're engaged in any workflow where you need to frequently move between tabs of different profiles it feels very unwieldy. For example, I'm frequently logged into office365 with my standard profile (where I'm referencing some document) and also logged into Azure or Intune with my admin profile (where I'm taking actions based on the previously mentioned document). The inability to use the standard ctrl+tab shortcut to tab switch is a huge pain - instead I have to remember a positionally variable shortcut, and if I'm throwing a third tab into the mix need to add the cognitive load of thinking about whether I can just switch tabs or need to switch workspaces instead (and also remember the direction of the workspace switch).
I think the reason people are specifically asking for something like Firefox Containers is because it's the best of all worlds. A browser can still implement something like spaces (which Orion does have in the saved windows feature) but allowing per tab accounts lets you do way more. If you look at Zen browser, it's firefox based but adds a feature like spaces. You can tell Zen that any tab in a space should open a specific profile (so at this point it still works exactly like Arc) but you can also override that. At that point, a space is just an arbitrary grouping of tabs which can optionally default to a specific profile but isn't bound to it because profiles work at the per tab level and not at the space level.