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Multi-Account Containers[1] is a Firefox add-on that lets you separate your work, shopping, or personal browsing without having to clear your history, log in and out, or use multiple browsers. It's an improved version of the Containers feature built into Firefox Nightly and available to advanced users by setting certain preferences (see below).

[1] https://support.mozilla.org/en-US/kb/containers

FF supports containers used by that extension natively; I think the extension basically adds some UI around it.

You can (one-time) set up domains to always load in certain containers, that works well for me.

I’m confused by this as well. It seems like a feature I would like to use without an add-on but they don’t seem to make it easy to find.