Strong agree on multi account containers. Keep in mind though if you disable them they drop all settings, unlike every other add-on ... ever. Bug is three years old but maybe we can push it over the top: https://github.com/mozilla/multi-account-containers/issues/1...
There is also no way to rearrange your containers aside from deleting them and making new ones in the desired order. Since I am using this for o365 administration it is a little annoying that I can't keep them in alphabetical order to find them easily.
Are you on the latest version? I can rearrange them on mine. If click on "manage containers" there is a gray on gray bar on the right. If you hover it, your cursor should change to an arrow to rearrange them.
That allows for visual rearrangement of that particular menu, but as far as I can tell the new tab button's list does not change and the extension keyboard shortcuts are still limited to the first 10 containers, which are bound by their creation order.
A non-sanctioned way to mitigate this might be achieved by editing containers.json, but I'm wary of inviting sync shenanigans.
This is something that I don't like about firefox. They have a ton of cool stuff, but I feel that they are always lacking a few things.
What I always give as an example, is how to add custom searches (Amazon, Reddit, HN, etc), you save the query url and add a keyword. Works very well to type `rdt something` and have the results. But: there's no option in the menu to see all keywords/search engines you have registered.