Due to the WeEX transition, I tried this addon as a replacement for openwith, only to find out it even works more reliable than the former. My only complaint is the the unnecessary "one last step" pop-up which randomly occurs despite the installed native-client. But since this is deactivatable in the options, a 5/5 is due.
Works great, good replacement for mpv-youtube-dl-binding , which won't work in Firefox 57 anymore. @dinko_dimitrov This is also the reason why the native client is needed, the new Extensions based on WebExtension technology has no OS level access.