Forgot to post a comment after the whole debacle a few months back.
Stay away from this extension, I moved to TamperMonkey.

The manage user scripts for Popup Mouseover is missing. This is a broken addin missing functionality.

Every script broke.

The old version was FANTASTIC!!

The new version doesn't work with my 3.17 scripts.
GM >=4 can't read scripts from the filesystem. So crap. So you have to use the crappy GM>=4 code editor.
The new GM also can't seem to access window variables, I can't be bothered to figure out why because I wouldn't want to use the GM4.x code editor. The Violentmonkey workaround I found online of serving the script from a local webserver sucks. It's a stupid and useless pain.

WTH!!!!!!!!!!!
This is the worst update EVER!!!!!
GM >=4 sucks and I'm sticking to 3.17.

If you want to run GM 3.17 this is what you need:
extract
https://download-installer.cdn.mozilla.net/pub/firefox/releases/56.0.2/linux-x86_64/en-ZA/firefox-56.0.2.tar.bz2
to /opt

run it like this
nohup /opt/firefox-56.0.2/firefox --no-remote -P foobar-profile 'http://foobar.com/' >/dev/null 2>&1 &

Go to preferences tell firefox to never update.

Disable GM 3.17 update
about:config
make new false boolean value named
extensions.{e4a8a97b-f2ed-450b-b12d-ee082ba24781}.update.enabled

Install GM 3.17
https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/versions/?page=1#version-3.17

什么情况？脚本大部分失效不说，插件连界面都简化没了，没有更新管理界面，每个脚本的开关还要点开二级菜单才行,只能装Tampermonkey了

It's a hurge work to change!
A lot's of people changed to Tampermonkey, so am I!
Bye~

4.x version broke the scripts.

Time to uninstall and change to working addon.

With the installation of 4.1, all my user scripts are gone, and there is no way to add a new one from the backups I had because the new user script button doesn't work 90% of the time (you have to click and wait up to 30 seconds to get it to pop up, and most of the time, it never does) and when it does, I can't save anything. This is on 57.

Where is the quality checking before breaking our web experience? Moving on to another scripting engine. Good grief.

all my scripts disappeared after updating to version 4.++

if this happened to you a fix is to reinstall the old version again

https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/versions/?page=1#version-3.17

you will find your scripts back :)

for developer you need to test you addon before releasing a major update

Do not use THIS. Use Tampermonkey - it's much better!!

Не используйте ЭТО. Используйте Tampermonkey - он гораздо лучше!!

I was forced to upgrade to GreaseMonkey 4.x with Firefox 56.0. My scripts stopped working. I wrote them, so I looked into fixing. I looked at the GreaseMonkey web site. I saw the general notice about the changes but could not find specifics. Didn't trust what I saw when I saw notes like "this script" hasn't been updated in years.

Fix up documentation. Provided example scripts with before and after changes.

Give warning if it doesn't work in Firefox 56. I tried Firefox 57, but that caused other problems for me.

Was able to turn off auto updated of GreasMonkey before downloading 3.17 version of GreaseMonkey. add ons > GreaseMonkey entry > more > click on off selector. See below for the link by Anonymous user e7bdbb to the download.

Robert
PS. There is snow on the ground as I write this :-(
PSS. FYI. Tampermonkey gave me a compile error on "for (var elmForm, i=0; elmForm=document.forms[i]; ++i)".