What are some alternatives?
Hover Zoom - Enlarge thumbnails on mouse over.
ezLinkPreview - Browse more efficiently: Preview links and images without leaving your current tab.
Thumbnail Zoom Plus - The official site of the Thumbnail Zoom Plus add-on for Firefox
ImageZoomer - Zoom images inline on the current page by holding the right mouse button and scrolling the mouse...
Mouseover Popup Image Viewer - Shows larger version of thumbnails when moving mouse cursor over thumbnail.
ImgTip - Shows image popup when hovering link that points to an image file or to image preview page.