Firefox button replaces the native icon. If I double-click the native icon, I can close the window. But even if I double-click the Firefox button, I cannot close the window. On Windows 3.x and Windows NT 3.5, window didn't have close button. Some users who started using Windows since these versions may be still using double-click on the icon. For them, this bug is very inconvenience. On the other hand, the Firefox button is more important than the system menu which is opened when user click the icon. So, the double-click could be just a miss-operation. I looked for the discussion about this issue, but I couldn't find it.
This is most likly a dup of bug 575166.
Opps my mistake.