Sorry, it does not work as I thought it was supposed to.
I installed it.
I have as one of my home pages a gmail work account. I assign it to Work, click on always load into work. When I restart Firefox Nightly it has not opened it in the Work container.
I tried setting the work container as part of the home pages, but that did not work either.
I do not know whether it is Nightly or some other problem. Both google emails accounts open correctly, but neither is separated from the other by category.

Now, after removing the add-on, When started Firefox Nightly yet again, I got a dialog that asked me into which container did I want to load the tab, and whether or not I wanted to load it into that container always. I would have given it a lot higher rating but for this strange "feature"