I posted on this a few months ago, preemptively. Didn't think to post again a couple weeks ago, when Firefox 57 became a reality. FF extension interface was completely rewritten, and ALL extensions had to be rewritten, to continue working. Greasemonkey HAS been rewritten, but, not completely/successfully.
Not a big problem, though, because there IS Tampermonkey available and working, instead. We've been using it on Chrome and other browsers for years. I came to prefer it over Greasemonkey.
After installing Tampermonkey, be sure to either remove HSX scripts in Greasemonkey, or remove Greasemonkey entirely (best idea, unless you have some specific need) & visit the Greasyfork page to reinstall all scripts into Tampermonkey.
https://greasyfork.org/en/users/76654-edzep