Things to try include: 1. Clearing your DNS server settings ( s e r v e r g u y . c o m / k b / c h a n g e - d n s - s e r v e r - s e t t i n g s - m a c - o s / ). 2. Clearing your /etc/hosts file. You can prepend lines with '#' to comment them out 3. Checking your system firewall by going to Apple menu > System Preferences, click Security & Privacy, then click Firewall. Make sure that outgoing connections are not being blocked. 4. Changing Firefox's proxy settings by visiting 'about:preferences#general', opening Network Settings, and clicking 'No proxy'. 5. Reinstalling aforementioned antivirus software.