Greasemonkey 4 is incompatible with previous versions due to changes made in Firefox v57.

https://www.greasespot.net/2017/09/greasemonkey-4-announcement.html

The changes shown in https://www.greasespot.net/2017/09/greasemonkey-4-for-script-authors.html need to be implemented to make it compatible with the new versions of Firefox and still Chrome