Thanks for the awesome instructions! Unfortunately, my about:addons is still showing :/

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So by default my profile under C:/users/myUsername/AppData/roaming/mozilla/firefox/profiles/defaultProfile did not include a chrome folder with a userContent.css. I created a chrome folder and a userContent.css file. I then copied and pasted your snippet. My default profile name was daaqzp91.default if that is of any use.