Starting from Android 6, all apps need to request for external storage access permission in order to read and write. Firefox does not have this permission. However, if you are using stock S7 rom (Touchwiz), then go to dev option and enable "force allow external SD access" and try again. If still fails to download to external sd, use 3rd party download manager instead.
I did as said, but for some reason, it won't work; im on the stock rom with latest patches, and I restarted my phone, and it still does not work; could it be a path error? When I use ZArchiver, it lists my sd card as /7DE2-D72A/, but inputting that, restarting my phone, and trying again still doesn't work ill float this over firefox support and hope someone knows how to do this. thanks for the help jychung93