Greasemonkey has been a good extension but hasn't had an update in 3 years now and it's showing. Some things are not working: recently wrote a script I couldn't figure out why it wasn't working as it was not at all complicated and looked all fine, so to check my sanity tried the script out in Violentmonkey where it worked fine. I think over the last 3 years as Firefox has had updates it may have affected compatibility for Greasemonkey. I would recommend using Violentmonkey as it is actively maintained and likewise open source.