Another new day, another new break from Google’s youtube control factory.
This is the best fix. The other one stops pasting in other apps. GrayFace said Here’s a selective fix for YouTube: greasyfork.org/en/scripts/12433-fix-paste-on-youtube You need to install Scriptish or Greasemonkey first.
Source: Why can’t I paste into youtube comments on firefox | Firefox Support Forum | Mozilla Support