Really fantastic add-on that boosts productivity. Allows you to compartmentalize your work tabs, social media tabs, banking tabs, whatever. This add-on keeps the cookies separate between tabs of different types, too, which is really great - I can have a Google Work tab open (with me logged in at work), and a Google Personal tab open with my Google Drive handy.
Importantly, you can specify that certain websites should always open in a certain type of tab (e.g., Asana should always open in a Work tab), which is invaluable for keeping things separate.
I also like the ability to sort tabs by container (move all your Banking tabs next to each other, for example), as well as the ability to reopen all tabs of a certain type in a different window. You can also create custom categories, etc. If you're anything like me, you have a million different tabs, and being able to pull out all my Work tabs all at once is a huge lifesaver for me organizationally.
Well done, Mozilla! -A big fan.