BLOCK ANY WEBSITES AS AN ADMINISTRATOR

The website that you want to block is         www.facebook.com. then

1. Open the hosts file with notepad. (C:\Windows\System32\drivers\etc\ )
2.type 127.0.0.1 www.facebook.com below the 127.0.0.1 localhost
3.save the file.
4. open Mozilla, type www.facebook.com
and hit enter. Now it will show page not found so user can not access the facebook.com

Update:
Windows Won't allow you to edit the hosts file from this location C:\Windows\System32\drivers\etc\ .
So copy the hosts file to desktop.
Edit as mentioned above.
Save the file.
Now copy the saved file and paste inside C:\Windows\System32\drivers\etc\.