Don't worry - there was an update to YouTube UI, it breaks the extension for some users, the fix is already submitted to be reviewed by mozilla, it usually takes around 24 hours to be accepted. As soon as you see that the version 3.0.0.9 is out - everything should work again

Fear not - YouTube's UI update has caused the extension to malfunction for some users, but we've already submitted a fix for review by mozilla. Typically, it takes around 24 hours for approval. Once version 3.0.0.9 is released, everything should function as normal.

There was an update to YouTube UI, it breaks the extension for some users. Fix is submitted for review, as soon as Mozilla team approves it - you'll have it. Takes ~24 hours, sit tight.