There is a preference in the Options window that controls this.

Open the Options window (click the Firefox button, then select Options). In the Options window go to the Tabs section and make sure the preference "Show tab previews in the Windows taskbar" is selected.

I did that earlier with no effect. Still had only a single preview for multiple tabs open. Think you may need to close Firefox and restart it.

I'm having the same problem! I'm using Firefox version 13.0.1 & the Windows 7 taskbar is only showing the current tab & I HAVE check-marked the tabs settings that says for it to show the preview & I've restarted Mozilla just in case & it's still showing 1 preview. Please help me. Why is it doing this? I hope someone can help. Thanks.