It's what I was looking for since a long time.
It works great and increase the flexibility of Firefox in a great way.
Now I can have multiple accounts for the same website all opened using my favorite browser.
That's great for work, where I need, for example, two separate logins for Dropbox.
Thus far, it worked great.
The only function I'd like to be developed, is the possibility of 'converting' one opened container into another... but I believe it's not straightforward, since one of the feature is to grant isolation between them...