Windows update and Firefox
On February this year, in one of the regular Windows updates, a Microsoft .NET Framework Assistant firefox extension was included without permission. This update adds to Firefox the ability for websites to easily and quietly install software on your PC. It is very difficult to uninstall but slowfox has excellent step-by-step instructions to disabling it.
Apparently it doesn't affect all PCs, but check out slowfox's post to make sure. This page has some more information on the matter.