This is a very simple extension that adds the song currently being played in the browser by Google Play Music to your library when you click the icon. I often listen to stations to discover music and got tired of having to search through multiple desktops and Firefox windows to find the Google Play tab to add a song to my library so this is a simple shortcut that works across multiple open Firefox windows. Here is a gif of the extension in action:
Right now the extension is very simple and barebones because it does everything I need. However, I can see it being useful to have the option to add to different playlists so if there is demand for it I could implement it; I'm also open to pull requests.
Currently only works on Firefox, I can work on porting it to chrome if there is any demand for it.