Exactly what I was looking for. I like having YouTube videos for music in a separate window while working, and this lets me control the music in my primary window without distraction.

Would love to see this incorporated into FireFox so that it supports all websites playing media, but until then, thank you for this addon!

One issue: the popup window seems to get confused on YouTube when the next video automatically starts playing. The window shows the previous video name and is missing control buttons.