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