YouTube™ Enhancer Plus
Warning: YouTube™ Enhancer Plus does not support the Material Design version of YouTube™, and since Mozilla has planned to disable XUL add-ons like this one when Firefox 57 will be released (source) I have decided to stop its development, so I highly recommend you to use Enhancer for YouTube™ instead.
YouTube™ Enhancer Plus is a Firefox add-on that improves your YouTube™ experience. It allows you to automatically play videos in HD or any other preferred format, disable autoplay for videos loaded in background tabs, play videos in cinema mode for a comfortable and distraction-free watching, repeat videos over and over without clicking replay, change YouTube™'s style using custom themes, and more...
One of the most appreciated feature is the possibility to remove ads from videos, whether automatically if you don't want to see them at all, or manually if you want to only remove ads that don't interest you and want to permit channels you're subscribed to to earn revenues from those ads so that they continue to produce great videos.
Another feature that is really appreciated, mostly by multi-tasks users having a large screen, is the sidebar. YouTube™ Enhancer Plus makes use of the built-in Firefox sidebar to allow you to watch your favorite videos while doing something else, such as surfing the web, chatting, or playing online games for examples.
YouTube™ Enhancer Plus is one of the featured add-ons for Firefox, so if you want to give it a try, click the button below to get redirected to Mozilla's website, then click on the green "Add to Firefox" button to install it.