To use it, you just have to keep the cursor over the link for a second, and then place it over the little Cool Previews icon that appears to the right. This opens a tiny window where you can preview the site. Although you can't browse the entire content of the webpage, you can still see what's in it, and decide if you want to enter the site.
Cool Previews is an extremely useful extension for any type of user. With it, you can avoid malicious websites before accessing them. If the site isn't what it says it is, you'll know it beforehand.
Unfortunately, this extension is no longer being updated, so it might not work with future versions of Firefox.
The developer no longer offers support.