The idea of multi-account containers is great and the context separation between containers works, so far, reliably.

However, two side issues turn out so cumbersome that the gain of clearity and ease this add-on could potentially enable is hardly effective in practice:

(1) If you deal with many dozen open tabs, you may frequently enploy the % shorthand in address bar to quickly find a particular tab. Alas, that shorthand doesn't work across containers, so it gets close to useless unless the searched-for tab resides in the same container with the current tab. In order to search for any other open tab, you (1a) first need to remember its context container, then (1b) find an arbitrary tab belonging to that container, then (1c) perform the tab search there.

(2) Even with the "sort tabs by container" menu item (which has to be invoked manually each time), the visual overview provided by the thin colour marks at tabs is limited. The UI would be much clearer if containers were, or optionally could be, assigned to browser windows, making all tabs in that container appear in the associated window and nowhere else.

This (2) would also help a lot in dealing with (1), as finding a tab belonging to a particular container (1b) would be as easy as finding the corresponding browser window. Given that you typically use much fewer browser windows than tabs, even (1a) is not a big deal anymore as you can perform the tab search once in every browser window (i. e. container).