The whole reason I don’t use Internet Explorer except on rare occasions is that I don’t want website add-ons to automatically install without a little bit of fuss. I would expect this “ClickOnce” support would make accidental installation of malware more likely. No, thanks. I disabled it until I absolutely see a reason that I need it–which is my normal policy with Firefox add-ons.

Security Fix – Microsoft Update Quietly Installs Firefox Extension.