The temporal workaround 4.9.10b2 makes it just worse Rated 3 out of 5 stars
before installing the 4.9.10b2 I got the same issue as the most people - 0 bytes or just not downloading anything.
After installing 4.9.10b2 the situation is that there is no recognizing anymore - the tool bar button stays greyed out, only the context menu is available.
The widget button within youtube sites doesn't start a download too.
Just clear your browser cache and reload the yYouTube page.