Whatever icon is displayed on the file is the program that is used to open that file. Internet links and a few others would have the Firefox icon.

Thanks very much for the reply. They show up on a wide variety of icons - the icon for my router settings, mp3 files, video files, folders, jpeg files, and video game files. They have never been there before - I don't know why they have suddenly appeared. Is there any way to remove them? They look like the attached image.

I had thought it was the Firefox icon alone. Now I see it is more. I've called the big guys to help you. Good luck.