This extension kind of worked for some period of time. However, I don't think it works anymore. When it did work it would not allow you to switch the quality down to a lower quality. This was an issue when a video had a 2160p option: especially when increasing the playback speed the video would stutter. So, when I attempted to lower the quality it would automatically set it back to 2160p.
Automatically change YouTube video quality to HD resolution or the highest available one
This extension automatically changes the YouTube video quality to a user-selected high-quality resolution