The only way I can add a script on Firefox 54 is to copy paste it. URL importing doesn't work, gives this error
Oops, you haven't got any script yet.
{ "url": "https://greasyfork.org/scripts/1317-download-youtube-videos-as-mp4/code/Download%20YouTube%20Videos%20as%20MP4.user.js", "data": "" }

好

Works great on FF Nightly 55 (2017-05-29), except for some small nuances, like the "Export to zip" not working, and unlike Tampermonkey it is open-source.

More smaller than Tampermonkey + under permissive license. Greasemonkey on this moment not supported webextension-api and must be rewrited from scratch for support Firefox 57+ (as of many other extensions).

Currently i use Tampermonkey.. someone has suggest to test this... what's the differences ?