After a thorough uninstall of Folx from my mac system, I was puzzled when Safari showed that FOLX 5 was in my plugins folder. I went to the plugins folder via Finder and nothing was there. I double checked that the plugin was indeed not in the Preferences > Extension Tab where I had uninstalled it months ago. Both showed no results.
I enabled the Safari Developer Menu: Preferences > Advanced Tab > Show develop menu in menu bar. Then went up to the Menubar with Safari in focus: Develop > Empty Caches.
That did the trick for me. Hope this helps someone out there.