You appear to be using Kaspersky security software according to that last screenshot, so it is possible that Kaspersky is blocking this website and not Firefox. *https://support.mozilla.org/en-US/kb/error-codes-secure-websites ---- Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems. *https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/ *https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10