The problem is with the cache and the way Firefox handles data. Please press Ctrl + Shift + Delete and clear everything from Firefox from the beginning of time, then open a Command Prompt and type the following commands:
net stop DNSCache // wait for the process to complete.
net start DNSCache // wait for the process to start
ipconfig /flushdns // this command clears the DNSCache pool.
Try to load Facebook again. Most likely this was a DNS problem.