This is because the unity modules aren't installed, it took me a long time to track this down. See https://bugs.launchpad.net/ubuntu/+source/unity/+bug/1313353
for the same issue in other apps.
The solution is to install both gtk modules.
sudo apt-get install unity-gtk2-module
sudo apt-get install unity-gtk3-module
Then log out and back in and the global menus should start working again in all your apps.