'''UPDATE (4 AUGUST 2021):''' Problem solved. Disregard original question below. '''UPDATED''' Yesterday, the websites '''jwz.org''' and '''dnalounge.com''' (yes, the two websites run by one of Mozilla's conceivers) stopped working on Firefox. They just keep saying "Connection Timed Out", despite every other website loading fine. I went through all the add-on-disabling and troubleshooting steps emphasised [https://support.mozilla.org/en-US/questions/1344977 here], but it hasn't solved the problem. This happens in both Firefox desktop and mobile for Android, but works fine in other browsers. Again, I went through all the steps of disabling add-ons/plug-ins and operating in Troubleshooter mode.