Thumbnail Zoom Plus is a Firefox plug-in which shows a full-size image pop-up when you hover over a thumbnail or image link. When you hover your mouse over a thumbnail or an image or video link, the add-on displays the full-size image or video still-frame in a floating window. Supported sites include Amazon, Bing, Facebook, Flickr, Google, IMDb, LinkedIn, Netflix, Pinterest, Reddit, Tumblr, Twitter, Yandex.ru, YouTube, Wikipedia, WordPress, Yahoo Images, and many more.
Version 4.0beta1 on Firefox 49.0.
I can no longer go past the first image of Imgur albums, as pressing the arrow keys does nothing. The image also shows a blue border instead of green.
Test link: https://imgur.com/a/4f3XB