I tried all the remedies out there.
Then I tried disabling AVG, and all my browsers came back.
It was AVG!
I had been running Symantec Endpoint Protection, and in addition, had AVG installed with its active protection turned off. That way, in theory, I could run an AVG scan at will.
Somehow the active protection had gotten turned on. So my guess is that it was in conflict with Symantec.
Then after it was all fixed, I ran across this when looking at Chrome settings:
Chrome detected that some of your settings were corrupted by another program and reset them to their original defaults. Learn more