I posted the following comment on MacFixIt:
I can add QuickBooks Pro 6 to the list of older apps that don’t perform with full functionality under Leopard. While QuickBooks starts up fine, the menu bar selections at the top of the screen stop responding to mouse clicks as soon as I bring up a checking account window. While running QuickBooks under Leopard disables the menu bar, the icon options just below the menu bar continue to respond normally. To close QuickBooks, I’m forced to use the keyboard shortcut, Command-Q.