Show tab previews in the Windows taskbar: When you move your pointer over the Firefox icon in the Windows taskbar, Windows will show you a preview of the Firefox window. With this option check marked, Windows will show you a separate preview for each tab.

This used to work. It doesn't anymore. It works on my laptop, not my desktop. I've clicked this option and relaunched Firefox, but it doesn't help. The taskbar preview works with other programs.