Thank you for reporting this. Did you open a page in the newly opened tab? The newly opened tab starts as "default" but switches to the container just before loading an external site. My add-on cannot display default firefox pages such as the new tab page or any "about" pages in a container. At least I haven't found a way to reliably do this yet.
If it doesn't work for you, I would be grateful, if we can have a chat on the github issues page https://github.com/chronakis/firefox-sticky-window-containers/issues so that I can solve the problem.
