Workmate.io

Any Internet person who has walked the roads of the web long enough doesn't need this add-on recommended to them.

Very complete and quite powerful, what you can do with this is a complete game changer in your experience with your favorite websites.
From sites as old as Neopets to modernized ones like YouTube, the way you interact with many sites can be changed for the improvement of the user's quality of life.

Combined with specialized sites or a good user guide, anyone can install scripts (and even modify them) in a very simple and accessible way.

This is a tool that cannot be missing for a netizen. Obligated.