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I've tried it in Tampermonkey and Greasemonkey but couldn't get it to work. Shift-click and Ctrl-click has the same effect as a regular click.
Could you perhaps elaborate on how you are using it?
With Tampermonkey it works with Chrome and Firefox. But at the moment, the site with the playlist has to be updated (e.g. with [F5]) before using the script. Then you should see multiple marked tracks, if you click at them with ctrl-/shift-click.