UPDATED MAY 8, 2021...
PLEASE READ: This awesome addon will be fixed very soon! I contacted Dishita and he gave this excellent response:

I am aware about the recent code changes by Youtube and the resulting No button display issue. A fix is already uploaded to the Firefox team for review and approval. The updated addon should soon get published. It will get auto-updated to v16.10.

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I have been using "Easy Youtube Video Downloader Express" for nearly 2 years. It is seamlessly integrated with Youtube and makes downloading effortless. When the latest upgrade came out, I felt compelled to make a donation to express my thanks for an excellent product. THANKS!