I was doing some experimenting with mta-sts (my blog) which required a webserver and new hosts. None of which worked in firefox until i had tls per host.
After swearing a lot, i even considered a server reboot but a hard restart of the web server eventually got me a site that worked as expected even if firefox kept refusing to serve the site.
The thing about mta-sts is it serves one file and firefox can read it it does the user no real point in visiting such a site unless your experimenting like i was.
Once the first was done the other three zoo domains got mta-sts.