I tried your new script using Greasemonkey 4.13 in Firefox 138.0.1 on macOS Ventura. The import seemed to go OK, but I don't see any highlights anywhere.

I see an error in the console:

Script error in [Greasemonkey script http://example.com//IMDb-Lists-Highlighter; version 1.0]:
ReferenceError: GM_addStyle is not defined

GM_addStyle was not in the original script.

Note: the import of my largest list (3,834 titles) is much slower than when I manually imported it using my hacked script. Are you "automatically" importing by scraping the list from the My Lists page? That is, click the list and then traverse through it?

There are two ways to import lists. Either automatically (just check the box next to the list name and then click Start Import) or manually via the CSV file for the respective list (to do this, click the check box next to the list name and select the CSV file, then click Start Import).

Automatic import imports the lists directly from the IMDB lists page in the background, scraping the content one by one.

Only custom lists and the watchlist work automatically. The ratings list and check-in list don't work automatically, only manually!

Automatic import takes significantly longer than manual import, but then it's automatic ;) The longer the list, the longer it takes.

Regarding your error, I can't quite reproduce it. Doesn't anything work at all? I've tested it several times, and for my use case, it works perfectly. Have you tried it at a custom list with just a few entries? Manual and automatic import?

It might be because you're using Greasemonkey. I wrote the script in the current version of Greasemonkey and only tested it there.

I will have a look at later and see if I can make it work.

Try Version 1.1

I added a Fallback for Greasemonkey. It works for me with Chrome/Firefox and Tampermonkey/Greasemonkey/Violentmonkey.