I'm a fan of Firefox Multi-Account Containers, but I wish @mozilla gave it the #UX love and attention of a core product feature. (It's currently an official extension: https://addons.mozilla.org/en-US/firefox/addon/multi-account-containers/). For example:
- Too many UX entrypoints with differing functionality (address bar, long-press + symbol, extension icon).
- Sometimes settings don't persist correctly and you end up with a site loading in the wrong container.
It's a difficult UX problem that IMO needs to be tackled at the browser level.