Functional, but needs work. I've given 5 stars as I can see the potential with the plugin to solve the headache of YouTube's new obsession with forcing radio playback on video songs.

There are some basic improvements the plugin needs:

1) An on/off button for the plugin (it is time consuming to go into about:addons to turn it off for a song list)
2) Better detection of the Radio feature. Currently it also blocks playlists (which it should NOT do). It should only apply to URLs that contain the keyword 'radio', not to entire playlists.
3) If disabling playlists is intended functionality, make it optional. I still want playlists. I don't want radio.
4) A whitelist that allows me to turn it off for specific video URLs.

These are optional suggestions for improvements that would improve the experience:
1) Intercept and change the URLs on the search page, rather than when the video is loaded. Currently it "double loads" a video page which makes the experience jarring and wastes data resources on E.G. mobile.
2) You can do the above by doing a string replace of "&start_radio=1" with a blank string in the HTML, or on mouse-hover.

This plugin has good potential, but it needs to be improved. Looking forward to updates!