Please try flushing your DNS on your computer.

The Domain Name System (DNS) is the service responsible for converting your URLs to IP addresses that will point your computer to the correct server for that website. The IP addresses are saved to your system with the corresponding domain name so that the system doesn't need to send a request to the DNS each time you enter a URL.

In some cases, this saved information can become invalid and may start to direct your computer to the wrong web server to may even bring up Page not found errors. Flushing your DNS will delete the saved list of IP addresses and force your computer to rebuild the list, likely correcting any previous errors in the list.

Does this solve the issue? Please let us know.

After two days, the website suddenly reappeared, so I didn't need to try flushing DNS. Thanks anyway.

Glad to hear that the issue has been resolved. It is likely that the DNS cache refreshed itself, which it does periodically, and thus correcting the stored IP address.

Thanks for contacting the Mozilla support team. Please remember to mark this thread as solved.