Like others, this stopped working after the 94.0.1 update.

Fortunately the post by Carlos helped get it working again. His instructions that work are:

1. Disable the extension (NOT remove)
2. From Firefox Settings -> Language and Appearance -> Colors, uncheck the "Use system colors" and change the drop-down to "Always" in "override colors specified by the page".
3. Check a website that you've clicked links on to verify the color changed
4. Go back to the Firefox Color settings and change the drop-down to "Only with high contrast themes"
5. Re-enable the plugin

You should now see the clicked URL colors use the color you specified while the rest of the webpage uses whatever is determined by their style sheet.