Something needs to be done to make Mozilla dialog windows look more like real Windows dialogs. At present it seems that nsAppShellService creates a nsWebShellWindow, and nsWebShellWindow creates a nsWindow. However, the nsWindow is created with a border style eBorderStyle_window. Instead, in this case the nsWindow should be created with a border style eBorderStyle_dialog, or else nsWebShellWindow should create a nsDialog instead of a nsWindow.
*** This bug has been marked as a duplicate of 7372 ***
Changing component to Browser-General. (HTML Dialogs is going away, and this bug doesn't look suitable for XP Apps on quick glance.)