Multi-Account Containers
I've been using this since it was new. I'm very frustrated to say the UI/UX hasn't seen any significant update.<p>Containers are still synced separately, so any fresh install of Firefox will have the tutorial UX and the default 4 containers, even after syncing.<p>Also, address bar completion is still monolithic, making it very easy to accidentally open a site in an undesired container. That can be worked around by adding every domain to a default container, but most of the utility of containers is to use several for a single domain, <i>i.e.</i> multiple email accounts each with their own container instances.<p>Containers are a feature I find very valuable, but they really need their UX to be a core browser feature, not an addon.