Then, maybe the hosts files was changed (before DNS lookup, the machine checks for name to IP translation in the local file called "hosts", it's a text file you can open to look inside).
Another easy way to check if it's the OS's fault or the ISP would be to download a live (portable) install for Ubuntu, and boot from the live CD/USB. After booting from the install CD, Ubuntu will ask if you want to Try or Install the OS.
Click "Try" in the Ubuntu GUI, and you will have a full running Ubuntu Desktop, with Firefox included. From there, click on the Firefox icon and check the problematic website, without any Win10 or antivir programs.
All these will not install and will not touch anything on your disks or in Windows.
Once you end checking, unplug the USB (or install CD) and shut down the computer. Power up again to boot Windows.