I've been testing this extension out for a few weeks and frankly I'm fed up.
The idea is good but it's just not user friendly nor intuitive.

First off it's unclear whether or not every site in the same container can 'talk' to each other. For example: If I open a Shopping container on Amazon, then another Shopping container for Ebay, can the 2 sites share information with each other and profile me?

I tried looking for this question and answer online but came up empty handed. So to be on the safe side one would have to make a separate container for EVERY site they want to keep isolated. That includes separating domains from sub domains.

Second: Sites that I have assigned to containers don't automatically open in said containers. Navigating to the site via the address bar produces a message asking how I want to open the site. Selecting the "Remember my decision for this site" option does nothing, but perhaps that is due to my Firefox settings or other extensions that block cookies and trackers. You would think though that Firefox would make this extension compatible with their built in privacy options.

Third: It takes multiple steps/clicks to remove a site you accidentally add to a container. For me the "Manage Extension" button is cut off (regardless of how zoomed in or not I am) so it took me even longer than others probably to figure it out.

Many people seem to love this extension but frankly there's got to be a better, more hands off approach to stopping sites from tracking and profiling you. If you aren't looking to have every domain and sub domain separate maybe try it, but I will be looking at other options.