I'm a longtime user of the Chrome version and just moved to Firefox. YouTube's stock layouts for streams with chat are terrible enough to make this extension one of the essentials. I've never had problems with it, and the dev is quick with any necessary fixes (due to YouTube changing things).

In case it might save someone some trouble with initial setup, make sure you right-click the extension and enable Always Allow on www.youtube.com. I spent a good few minutes trying to figure out why it wasn't working before seeing that step mentioned elsewhere.