Thank you, but I think I found the underlying problem. Since the problem occurred both with my cable router and with my iPhone hotspot, it had to be in my computer. While the computer rebooted, an error dialog box flashed before my eyes. I didn't see much of the content, but it made me guess that the SSL software must have crashed. Among all the web sites I listed, FR is the only one that uses a non-secure HTTP link.
Something vaguely similar happened once before, and I attributed it to the VPN software I use to access some websites. Prior to this event, I had inadvertently left the VPN on when I put the computer to sleep, so I think that a bug in the VPN software failed to properly release some network resource back to the bare Mac.
Everything works now.
Good that it is back! “SSL” is controlled by the browser for websites (Safari if Mac “default”). Maybe the fresh instance of Safari after reboot fixed it...my elderly Mom had Safari go “wonky” the other day for the 1st time ever...I had to “Team Viewer” remote into her Mac (am 3 hours away) and “force quit” Safari & restart it.