I stopped getting FB about 2 hours ago, then whilst trying to find fault saw that windows update wanted to install new win 10; so I left it to run. Came back, still no FB. I tried it via google, - i tried it via desktop icon, tried it via address bar - nothing - same answer 'server cannot be found.' Would try to uninstall ap but cannot open apps page either. Will try via control panel. Its not in there.

Server not found

Firefox can't find the server at en-gb.facebook.com.

Check the address for typing errors such as ww.example.com instead of Example Domain
If you are unable to load any pages, check your computer's network connection.
If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web.