My addon can work correctly in some previous version (I don’t know exact version). For current version (59), it cannot work correctly. When I scroll down to the bottom of popup page of addon, whole content dissapears. I try write new addon, and I can see that, if we set css attribute position:absolute/fixed, this bug will appear. Addon for test: https://addons.mozilla.org/en-US/firefox/addon/smart-emoji-keyboard
I tried the extension you linked to and it worked fine for me. Did you find what the problem was?
Did you scroll to reach items at bottom? You will get the first screenshot (whole emojis disappear, only black space there). It works fine on Firefox 56, from 57+ that bus fires!
Works fine for me. I tested on Nightly, on Mac OS. It could be different with a different setup.