Why won't Firefox recognize application keyboard shortcuts created in Mac OS X System Preferences?

I go into System Preferences, open the Keyboard & Mouse pane, and create keyboard shortcuts for various Firefox menu items in the application keyboard shortcuts section. When I load Firefox, the menu items now appear with the appropriate shortcut listed, but pressing the key combination merely flashes the menu header (e.g. for “Add-ons” it only flashes Tools) without performing the appropriate behavior. I’ve checked the key combinations to make sure they’re unique, checked in other applications and the shortcuts work, and clicked the Firefox menu items to see that they work. There’s just something about Firefox being unwilling to open the menu item when the instruction comes from the keyboard. Any thoughts? (I’m running Firefox 3.5.1 on Mac OS X 10.5.7)