tried your "MultiThreaded Download Manager" v0.7.1 on Windows 10 in Firefox (FF) v57.0.4 with ABP, NoScript, etc.

that version showing : "100%" downloaded, but progress bar remains as red-colored ! and %USERPROFILE%\Downloads\ folder has no downloaded file ! where is it saving by default ? !!! DOM / FF-Temp/Cache ?

after pressing the play button (shown next to filename), the file "Save as" window appears, but even after doing that, still NO file inside selected folder ! both HTTP & HTTPS based download fails to save into selected local folder.

EDIT: above problems are solved, after FF was restarted once :)

One more issue is : many secure website (for example: "https://www.example.com/") uses a common(multidomain) or primary/main/"www" or wildcard/* based SSL CERTIFICATE for all of their sub-domains. So when FF user has accepted such primary/multidomain/wildcard SSL-cert for their different subdomain (for example: "https://download.example.com/", "https://cdn.example.com/", ...) to view webpages and if FF user attempts to download file(s) from such HTTPS based subdomain, then files are not downloaded by this extension ! So this extension need to recognize those (multidomain/primary/wildcard) type of SSL-cert which is used by that subdomain (as user has already accepted that SSL-cert inside FF) or this extension need to prompt/ask FF-user to view+approve that SSL-cert (for that subdomain) to continue downloading process over encrypted HTTPS connection. The play-button which is shown next to file-name, can bring the "Save as" window, but cannot save file into selected folder !

EDIT: above problem still exists even after restarting FF once. And after restarting FF, now "Save as" window does not appear anymore for such downloads ! so such files are not downloaded !