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.
Streamable hosts videos. Thumbnails can be retrieved via their API.
Example Reddit post. Streamable page. API. "thumbnail_url" is the one you want.