"Unable to connect" only facebook

Hello,
Every time I try to go to Facebook I get a unable to connect message saying Firefox can't establish a connection to the sever at www.Facebook.com I use to be able to go to the Facebook web site with no problem then all of a sudden every time I tried to sign in it would tell me I had to download all these different php files before I could log in that happened a few times and then this happened. I'm not to computer smart but I've tried changing the firewall settings and Ive ran different virus scans but nothing has worked. *I only get this "Unable to connect" message when trying to go to Facebook website* No problems with any other websites. Thank you for your time and answers.