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.

Doesn't work at all. Probably need to be updated to fit on the new version of Firefox ?

已经不能用了，暴力猴可以用

!! WARNING !! Malicious code contains password theft and credit card theft scripts!!! $ 600 to me, this script did damage!!!

스크립트 설치 안되요

What a load of rubbish. No tutorial, no scripts work. Have to post your script online then... , user interface is flaky, Error messages are none helpful. Just a pain for something that doesn't work. Good concept, too much monkey business going on here!

I already download fire fox but it says I need to download it >:(

Sripts are not working, cant instal anything.

Straight up didn't work, at all. I've switched to Tampermonkey and NOW the user scripts actually work!