Firefox Multi-Account Containers

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It's an extension:

Each Container stores cookies separately, so you can log into the same site with different accounts and online trackers can’t easily connect the browsing.

A great idea for a feature if you ask me. For example, I have two Buffer accounts and my solution is to use different browsers entirely to stay logged into both of them. I know plenty of folks that prefer the browser version of apps like Notion, Front, and Twitter, and it's cool to have a way to log into the same site with multiple accounts if you need to — and without weird trickery.

This is browsers competing on UI/UX features rather than web platform features, which is a good thing. Relevant: Opera Neon and Refresh.