The following setting should be part of the default privacy settings:
lockPref("dom.event.clipboardevents.enabled", false);
You can take a look at:
https://support.mozilla.org/si/questions/969543
Particularly the comment from jscher2000 there.
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not worth it at best -> https://github.com/arkenfox/user.js/issues/1081#issuecomment-906560952
Thanks !