Since the menu change in Firefox to make the menus look like WinXP's themed style on all versions of Windows, rather a lot of people have been upset. Quite understandable, really, as the change has just made Firefox's menus look seriously lame on non-themed systems.
So much is the annoyance, that I've gone and started hacking nsNativeThemeWin.cpp to try and make it support theming menu components on Windows. It ain't pretty, but I've managed to fix some bugs with the toolbox and toolbar appearances already (which help a lot), and have got basic themed menupopup and menuitem support going. It lacks the classic appearance currently but, now I know my way around, that should be easier.