I've been a Firefox user for more than a decade and today is the first time I've had an issue that is making me reach out to the support community for help. I have one computer at a client in which Firefox just won't load certain websites. One of them is www.performance.gov the site loads fine on other computers on the same network, the problem computer can successfully ping the site and correctly resolve DNS. Furthermore, Edge, Internet Explorer, and Chrome on the same computer can also load the site perfectly fine - it's only Firefox that can't. I've tried to refresh Firefox, starting Firefox in Safe Mode, reinstalling, completely wiping Firefox from the computer (removed all of Mozilla's install, %AppData%, and %LocalAppData% directories; as well as removing all of the Mozilla and mozilla.org registry keys), then I installed a clean copy of Firefox 85 - still won't load the site. I've also tried changing Firefox's proxy setting to "No proxy", as well as disabling DoH - no dice. The problem computer is running Windows 10 x64 20H2, is patched to today (2/2/2021), and has been restarted several times between all the attempts to get Firefox working correctly. I'm really at a loss here. Does anyone have any ideas or suggestions? Could there be some kind of TLS or certificate problem on the machine itself? Like could Windows be set to only support TLS 1 and 2 but not 3 and Firefox is inheriting that setting? Idk I'm kinda reaching here but I just don't know what the issue is now. Thanks in advance for any suggestions!