Sorry to bomb you like this, but I don't have the time/energy to find where to file a ff container bug: twitter.com never opens in it's designated container for me. That is the only site I always have to "Reopen in container".
Same here: that's the only site that doesn't work. I wonder if it has something to do with the post-login redirects: it's impossible to say "always open this site in this container", because you've already been redirected to a different url.