After testing almost 20 different extensions, I finally found THE BEST one! Stop looking and just install it, and please read the instructions on the options page, you can even get all you already installed extensions just with a few clicks (no other extension can do this, due to WebExtensions limitations, but this guy found a way). This is the most configurable and easily the best context search add-on after the last Firefox update! I was using for a few years the Context Search X and felt sad it didn't work anymore on FF57 but this one right here is the best alternative for sure, trust me, I tried several others and all of them can't do what this special little marvel can!