i've done this before, and i think that it is done in the toolbar properties section. something about showing non-minimized windows. (can't check the exact names and locations as i am at work on a windows box
) try right clicking on the toolbar and looking through the options. to see if this is the case, minimize that mozilla window. does it now appear on your toolbar? if so, i will look when i get home today and tell you exactly how it is done.
hope this helps, and sorry about the lack of exact details.