The site could be temporarily unavailable or too busy. Try again in a few moments.
   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."

Hello, I am having trouble accessing to certain websites using my MacBook Pro. The screen shows: "Unable to connect Firefox can’t establish a connection to the server at www.ncbi.nlm.nih.gov. The site could be temporarily unavailable or too busy. Try again in a few moments. 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." Can someone please help? Thank you!