Most of the solutions blame the problem on security programs and suggest turning off components of web scanning. The problem is actually with a Windows 10 update. I spent several hours tinkering with Avast, finally uninstalling it, to no avail. The only thing that worked was rolling back the update. Since then, I've installed stopupdates10, a free utility to block these unannounced and unexplained updates. That's a better solution than having Windows break my computer. But I wish Mozilla would address the Windows 10 update problem, rather than continuing to repeat what in my experience has been pretty useless advice.