Earlier quoted context omitted.
Yes, I can still reproduce it on 64. I'm on linux though.
Pretty sure I figured this out, since I was able to reproduce it as well. It has to do with caching and something Whatsapp is doing to restore the page from local data most likely, as once you clear the local data for web.whatsapp.com, it correctly requests redirecting to the correct container. If I had to guess, I suspect that Firefox triggers the container switch dialog on the first network request, and that page i…
You can try this if it affects you.