This is so cool: I have to use multiple logins on single URLs, like Google for business and personal, same for Microsoft. Now, I can use them all without the need to use separate browsers. Now, instead of having different browsers for each purpose, I can use Firefox for everything.

And, yes, the implementation could be improved with little changes that would ease the use. Like "Open a new tab by Alt-Cmd-Enter" in the same container instead of no container. Same for "Undo close tab". But, that is minor.

Bottomline: super cool and useful. Go for even more!