Rated 4 out of 5 stars
in firefox v21 the titlebar-icon (main-window.ico) doesn't work anymore.
it is still where it should be (mozilla firefox/chrome/icons/default) but titlebar shows standard firefox icon.
I am afraid that this issue is due to a bug in Firefox itself. The paths to many files have changed in this version of Firefox due to changes to support Microsoft Windows 8. This may not be a bug, but a "design feature" since, possibly, Windows 8 may not have window icons and that now carries over to older windows versions when using the new Firefox. There also may be a new path into which the icon files can be placed to work. I do not know the answer. Reporting bugs gets you nowhere with Mozilla. I am afraid this feature is used by so few users that no one will report the issue and get noticed by the developers. I have no time to search through the many megabytes of source code to find out the truth to this issue.
All that I will do at this point is wait and hope that someone fixes this bug in the next version.
P.S. this issue is not limited to the main window, but all windows.
UPDATE: as per this discussion: http://forums.mozillazine.org/viewtopic.php?f=18&t=2708993
The new path is as follows:
C:\Program Files\Mozilla Firefox\browser\chrome\icons\default
Thanks for your support honx4amiga!