Clipboard may be cleared or modified by websites
document.execCommand('copy') allows websites to mess with the clipboard. We might want to think to either warn users when this happens or just disable that feature by flipping the
dom.allow_cut_copy preference. See: https://bugzilla.mozilla.org/show_bug.cgi?id=1012662 for the original implementation and discussion about exposing the preference.
Thanks to xiaoyinl who reported that issue in our bug bounty program.