It works for me on the latest Google Chrome (Version 84.0.4147.89 (Official Build) (64-bit)) and Firefox (versions 77.0.1 & 78.0.2 (64-bit), with Greasemonkey v. 4.9) on Windows 10.
Can you see the script's toolbar (buttons [Refresh highlight data] [Clear highlight data] [Help / About]) when you open imdb.com/mymovies/list ?
If you can see it, I suggest you check the help, then [Clear highlight data] and [Refresh highlight data]. Now any IMDb page you open or refresh should work.
P.S.: be aware that, while OpenUserJS (this site) offers an awesome service (and for free!), IMHO it has one annoying flaw: your login can timeout while you write a message or reply, so if you spend some time writing your text (as I did), you might lose it when you send it. Always copy your entire message before clicking [Reply]! :)
I upgraded to this from the older version in the other account. but it still doesn't seem to actually do anything for me I see no difference having this installed or not?
Is it broken?