I love OpenUserJS! I sign-in, write you a long reply and the site deletes it and asks me to login again... 🤦
Anyway, do you have any other IMDb related script or add-on installed? My script works for me on Chrome and Firefox. I suggest you to open a new tab, open the JS console, load https://www.imdb.com/mymovies/ , then copy-and-paste any errors and warnings here.
I'm not sure if the script still works with others, but it doesn't work with me. The top buttons (refresh, clear highlight.. etc) don't appear after refreshing Firefox and re-installing the script. (Firefox 67.0)