Avoid like the plague.
I used Greasemonkey before version 4, but ever since the webextensions were forced after Firefox 57, GM never got back to where it was previously. They tried to force new APIs without any backwards compatibility which was a terrible decision, and other decisions were made which were ultimately the downfall of this addon. And now it hasn't been updated for years...
It's just sad what happened to this once awesome addon.
Install another extension such as Tampermonkey instead, it's a billion times better than this.

WARNING LOT'S OF SCRIPTS WON'T WORK USE OTHER ADDON LIKE TAMPERMONKEY OR VIOLENTMONKEY. Review for version 4.11 now 31 months old. It refuse to install some scripts and that will get worse because firefox evolves. On top of that I wrote a local script. Grease monkey validates the name but use it own name. ( unnamed script 123455 ) and that can't be changed. When I made a seccond script I couldn't be saved beccause "unnamed script name space is in use. This addon should be removed because script writers are dependent on it and this one has become a disgrace.

非常好