Link Alert is a Firefox plugin which will change your mouse cursor when you point to a link, so that you will know what you are going to open/download after click the link.
This simple plugin will show you a small icon when you move the mouse over a link. Although Firefox will show the link details in status bar, but this plugin will let you know more details. For example, you will know the filetype that you are going to download.
You can also customize the link icons with image that you familiar, or add any specific file extensions. This lightweight plugin will not slow down your Firefox speed. I had tested it with my old PC and my Firefox speed remain the same before and after installation of this plugin.