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I guess you were using Greasemonkey or Firemonkey, which don't run scripts directly in the page context like Violentmonkey/Tampermonkey do.
The new version of the script is now compatible with Greasemonkey.
I was not aware, thank you vm for the info and the fix. (I was indeed using greasemonkey.)
The updated version still did not work for me in greasemonkey, so I just switched to tampermonkey. Thanks again.
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Not working at all for me, 7/5/21. Firefox 88.0.1 (64-bit). No other addons, just this script.