This is already a feature on Chrome but I don't see it working on Firefox.

On Chrome, when you have a stream playing in another tab and that tab is not focused for some time, the video quality is automatically reduced to save bandwidth and when you go back to the stream tab, the quality goes back to normal.

This doesn't happen on Firefox though. It might be a browser limitation but it would be great if the Twitch folks took a look at this.

Firefox users deserve dynamic quality switch too :)