Works great, does exactly what I wanted it to, except for one thing:
When the add-on is enabled, I can't play two videos at the same time (say, I have two YouTube tabs open for, say, a podcast with no background music and background music). I don't know why; I don't know if that's a bug or a feature, but if it's a feature, it's a bad one, and I'd at least like to see an option to disable that.
If it is a bug, please fix it. I'm adding it as an issue on your GitHub.
Keep up the good work, though. Again, I gave this a 4/5 for a reason.