An installation guide for non-developers would have helped. I did not understand why installing another client was needed for the extention to work. Maybe a youtube video with sound could explain step by step what each part does. Since all the changes with Firefox seem to be motivated with security aspects, It seems strange that the first "new" compatible extention demands that you install a n "extra client " out of an obscure package called "windows", found somewhere in the internet. And that client program does not even have a name, nor an install path, it just installs somewhere into your system.
Besides all that, the downloader still simply failed to download, even with the extention installing allright (It did check green). Then the first downloadable file, a mp3, produced just a "error spawn" message with a cryptographic text. That was 20 minutes in and I just uninstalled it alltogether. Why was downthemall or flashgot able to do what you'd expect even back in 2006?
I do not undersstand why people put so much effort into creating these AddOns and nearly zero into conveing their product to the user.
Really astonishingly bad user experience in 2017.