Could you turn on viewing of file extensions. The following article has the steps: https://support.microsoft.com/en-us/windows/common-file-name-extensions-in-windows-da4a4430-8e76-89c5-59f7-1cdbbc75cb01 The reason I suggest this is to see what their extensions are. Maybe .pdf or some other type of file got associated with Firefox. That's easy to change in Windows settings, but first you need to know what type of files they are.