Certain pinned icons on taskbar not combining

[gaz]

On Windows 7, this is how I have my pinned icons arranged on the taskbar.

 

 

When I click IE, it merges the new window with the IE icon, like this:

 

 

Firefox used to do this too. While not paying attention, I managed to unpin FF from the taskbar, and when I repinned it, FF's window doesn't merge like it used to, it opens a new window besides it:

 

 

How do I make FF behave the same way as IE on the taskbar? The other icons are fine and behave like they should. If I pin a new program to the taskbar, it behaves properly, just Firefox not merging icon and window.

Edited 26 May, 2011 by gaz