FLASH UPDATE (09 OCT 2021) : Youtube made code changes which broke the button display, a new version has already been submitted to Mozilla for code review and approval. You should get auto-updated to v 17.4 in the next 2-3 days. Pls. do not open a support ticket for this and follow instructions below to get things working temporarily.
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Pls. make sure you are using the latest addon version, When you update the addon lower resolution downloads may result in "https://www.youtube.com/undefined" undefined redirection issue, just retry the download and if that does not work Refresh the page - sometimes it takes to refresh two times and even clear browser cache and it should work. If the button is not responding make sure "no other Youtube related addon" is enabled in your browser.'
From time-to-time Youtube experiments design and code changes restricted to very few users, and if the above does not works, you must be facing the same. You can try:
A). Allow addon to run in the Private/incognito window and then open Youtube videos in a new PRIVATE Window (Firefox Menu -> New Private Window) and tell me if it solves the issue for you?
B). Sign out of your youtube account and refresh the page, also, try another video.