Probably the biggest reason for using Firefox. Don't have to switch profiles, etc and let's me have a single window where all tabs live;

Super helpful to manage multiple accounts across the same website, MS, etc so there's no overlap between the different tabs

very informative

Very good add-on, 6 points from 5

Absolut unverzichtbares Tool und Alleinstellungsmerkmal von Firefox.

Truly helpful add-on. Love it.

I couldn't imagine using a browser day to day anymore without this feature.

To @daze who asked about changing stuff once it's set to 'always open'. Click on the Add-on int he toolbar, go to 'Manage Containers' > choose the container which was assigned and then select 'Manage site list'. You can then remove sites from opening. It would be nice if it could be disable temporarily.