This extension is almost perfect for me. It is clean an minimal just like I like it. I only wish it included the ability to adjust the width. I use an ultra-wide monitor and there is still a lot of dead unused space to the left side of the video and right side of the comments.
The most minimalist and simple extension for this function that I could find on firefox after switching from Chrome. The other more popular one works but is more distracting to the eyes. This one works perfectly and does all it needs to do.
