FF window focus oddity

I've noticed some strange behaviour with the FF window's focus recently. Check this:

1) Launch FF.
2) Change window focus to another application (your web browser for example).
3) Roll the mouse over the FF menu bar. Notice how the different menu items get preselected upon roll-over.
4) Now left-click on one of these items. Notice that the window focus doesn't change back to FF and the menu item doesn't expand. This 'works' with the FF main window menu and the Filter Editor window menu.
5) Clicking elsewhere does return focus to the FF window, it appears to be just the menu bar that has this odd behaviour.

Excellent. This one got squished!