CoolPreviews is a very good Firefox Addon which helps in previewing links and images without leaving the page you are currently browsing. You just need to move the mouse over any link and and a preview window appears showing you the content. If you need to preview several links on the same page then you can hover over one link after another without closing the preview window to make it faster.
FEATURES
* Preview web links, images, and videos without leaving your current page or tab.
* Automatically subsearch Google, Wikipedia, and others
* Temporarily bookmark items to the right column with our unique “stacks” feature
* Instantly send links to friends and family with just a click.
* Effortlessly scroll Google Image search results on an “infinite 3D Wall” (Cooliris addon required).
FULLY CUSTOMIZABLE
* Control how the preview window opens, i.e. speed, activation method, etc.
* Control the sensitivity of the preview window
* Toggle previews on/off on any specific site (click our icon on your browser status bar)
You can download the addon from the official firefox addon repository at: