Thanks.
That's how it works now. Firefox Extensions don't have access to your local files. If you saved the file recently, I can be sure if it's a duplicate, but for your old files, the most I can say is if the file with this filename already existed, which is not enough, because some pages might have different pictures with the same filename, something like "image.jpg". Or some "1.jpg" file would prevent you from downloading another completely different file with this name.