Unfortunately you can't edit these xml files in firefox.
Good idea with backup, I searched the entire disk, for files with the same/content, but didn't get any results. Also, if I remove a file from the folder, firefox does recognize, that it's gone, so it must be the right folder/files.
Boy, this is really frustrating. I never had the problem under Windows XP, is it possible that Windows 7 has some type of security layer that is working against me?