:'(

Could you elaborate why it did not work for you?

In version 1.2.0 you had to have a playlist preloaded for playback controls to work.
Perhaps you had no playlist loaded?

Since version 1.3.0 UX is better: playlist is auto-generated on the fly -- give it a try!
More details at https://github.com/lidel/google-music-hotkeys/releases/tag/v1.3.0

As for using media keys, that will be possible as soon as Firefox implements missing API.
More details at https://github.com/lidel/google-music-hotkeys/issues/1

Update: since v1.6.0 you can customize key bindings, hope this helps!