Hi, Specimen 9999. Thanks for the quick reply.
Alas, the culprit does not seem to be any extension. I had only one extension running for Safari (1Password) and I tried disabling it per your suggestion, but I'm still getting the same kind of warnings from Avast's Web Shield. All of the warnings seem to be related to attempting to access kubows.com, forbition.com, fairtray.com, and volcanish.com. They all seem to be connected to activity by com.apple.WebKit.WebContent.
I'm totally puzzled. I have never gone to any of those websites intentionally, and I don't seem to have any cookies or databases on my system that are related to them.
Any other suggestions would be appreciated.