works from waterfox56 up to firefox nightly 66
my own version, its a little different as it loads
normal userchrome scripts from chrome/legacy
and
sanboxed(bootstrapped) scripts from chrome/sandboxed
and
xul overlays from chrome/xul (im using the jsm files from thunderbird 61, overlays were removed from firefox)
the whole chrome folder in your profile folder is internally exposed as
chrome://userchromejs/content/
why is this important? because chrome uris have no security limitations which mozilla has already or will impose on file uris
which makes its very easy to load additional files in your script , just add a subfolder and access its contents like this for example
chrome://userchromejs/content/sandboxed/myaddondata/icon.png
this way you can create or port whole addons very easily, not just simple scriptfiles
the utils folder contains util files for the loader to work