When you have "Warn before accepting a ..." and a site tries to set a cookie or display an image, Mozilla prompts you for setting the permissions. However there are some problems here: 1. The dialog box does not appear on the taskbar so there is no way to switch to it directly using the mouse. 2. The dialog box does not show up in the Tasklist (accessible with Alt-Tab) so you can't go directly with the keyboard. So the only way to get to that window is to minimize/close all Mozilla windows. The bigger problem is that the question/confirmation _BLOCKS_ all Mozilla windows, so Mozilla appeared to be crashed/stopped working.
Much the same bug as bug 71222. Modal dialogs do not get a taskbar entry on windows, or a menu listing within the app. But, it would be preferable if the dialog was window modal, instead of application modal (e.g., that other windows continue to fully work, but the window with the confirmation dialog is blocked on user input). (However, app-modal is the way that Nav4.x operated on win32).
No secondary windows should appear in the taskbar. See Windows Interface Guidelines, page 104. resolving as invalid, main issue is covered by bug 71222