First of all it must be said that the container feature was the reason why I switched from google chrome back to firefox. In the past I needed two browsers on my pc: one for my private gmail account and one for the school gmail account - now there is no need for using a second browser 
Although there are already some important containers, I would need an option to add new containers (with a random colour as icon). For instance, I worked in the summer holidays for a company, which also gave me a seperate gmail address. So I had to use the shoping container for the third mail address, which was quite anoying, because google uses your search history for displaying personalised advertisement.
BTW: I have the same issue on some other websites, where I need more than only one account.
Maybe someone could add this feature
