Cannot connect to Google.com or Google related sites (e.g. Gmail). Other sites are fine.

A few days ago my primary home computer (2010 iMac running Mac OS 10.12.6) could no longer access Google.com or other Google sites (exception seems to be YouTube which is still ok). When attempting to go to Google.com I get "Safari cannot find the server" if using Safari and "We can’t connect to the server at www.google.com" if using Firefox. Other devices on the same wi-fi network including Apple and PC products do not have this issue. Virus scans so far have not found anything. What else can I try to troubleshoot?