The menu item property "type=disabled" works fine with IE.
With Firefox it disables the menu (can't be clicked) but it doesn't grey it.
I've been trying to use "mm_changeItemProperty" on oncolor/offcolor, rawcss... but nothing works, maybe because of the complex css file I use in my page.
Many FF menu updates recently. Move up to the current version (5.748 - which you need to be at for support, anyway), and then let us know if you still need help.