Great addon. I once switched to Chrome, because of the HoverZoom addon and I didn't find one for Firefox, until I found this little gem. But: could you add a function for links to Flickr? I regularly browse Reddit, but many people there post pictures from Flickr, but this addon doesn't work with that site. I must be possible, because RedditEnhancementSuite does it. Maybe also for Imgur albums? Something like on hover the first picture is opened and by pushing the arrow-keys the pictures switch to the next. That would be awesome. You anyway get 5 stars from me :) Edit: There is also a bug, that doesn't allow to show a picture when scrolling down without moving the mouse.
Thanks for your review. Flickr.com links and imgur albums are already supported in the latest beta https://addons.mozilla.org/en-US/firefox/addon/thumbnail-zoom-plus/versions/ . This is planned for the official 4.2 release.
The issue about images not popping up when you scroll is actually a feature, not a bug. If you want those pop-ups, go to Thumbnail Zoom Plus preferences and turn on "Scrolling triggers pop-ups".