Hate to tell you that SCDL 2.21 is not working on the newest Firefox update (57.0) either. The problem is still the same with deactivated adblocker(Adblocker Ultimate). But I apreciate the idea and it is not harming the system at all. That's what the three stars are for. xD I can send you a screenshot of the closest I get to download a track. ;)
Just b.t.w: It is the exact same problem since the first time I installed it, my friend(might be more than a year ago). So it is maybe your AddOn or it could also be your description, that leaves us alone with questions like "Where the hell is it even supposed to be downoaded to?" or "Will there be a download popUp or sth. like that?". I think regarding the past you should think about some screenshots as a "Download-For-Dummies"-Tutorial, for example. Enough joking: I'm excited for the day on which I will read more than "SCDL currently downloading nothing" as its only true message. Whatever a message of succes might look like. ;)
hahaha. great review! unfortunately i'm not as active in support as i should be. but just now i made new major update. that should fix most common problems like. pup up blocking or newer Firefox versions. the download location is the one set in the firefox preferences.