There can be two mini icons per menu label instead of one now. A menu item can get up to three different labels. The first two are left aligned, the third is right aligned by default. HoldSubmenus is the new default for mwm and fvwm menus. When you move back from a submenu to its parent menu the submenu remains visible.
Menus can be visible multiple times at once.
Dynamic menus are provided by the special menu items DynamicPopupAction and DynamicPopdownAction. See the man page under AddToMenu for details. FvwmBacker is now page-aware, using a new configuration syntax. New FvwmPager options SolidSeparators and NoSeparators. The old Panels feature of FvwmButtons has been removed.