Is there any extension or add-on that works as a hover thumbnail viewer?

Hah, I just zoomed this site to 150% in my browser, it's easier that way. :D Same for rule34paheal and xbooru.

I use Imagus...does basically exactly what you are asking for...just hover over a thumb and it will bring up the full image.

Firefox: goo.gl/5QAWQJ
Chrome: goo.gl/EeKpFY