I really enjoy the ability to isolate sites with containers. Works great if you want to be logged into multiple accounts on the same website. Two or more gmail accounts open at the same time.

This functionality is wonderful when doing web development. Testers/developers can be logged into all the website roles at the same time if desired. Great when used in combination with https://addons.mozilla.org/en-US/firefox/addon/temporary-containers/

Best solution I found for managing multiple AWS accounts simultaneously.

Must have extention

Best innovation in browsing in a long time. Combined with Temporary containers is an amazing combination.

Please, add synchronization of containers

In times where privacy both matters and is at risk, it's great to keep your logged in sites in something that looks like a different browser (the containers). Besides it also allows to have different accounts on a given website and be logged to both of them at the same time, just in different containers. This is the best thing Mozilla could have made for browsing. A fantastic enhancement on Private Mode (which is a bit like a container too, but it does not preserver your logins, history, etc., whereas containers do.)