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.
That problem are you seeing? For me, both the link embedded in github and the reddit page thumb show a pop-up, at 640x400 resolution. Are you not seeing a pop-up?
Example
image: statue2
link: statue2 : glitch_art