That is exactly the expected behaviour of "seamless" mode where we only want to display the application windows of a guest OS on the host desktop:
With the "seamless windows" feature of VirtualBox, you can have the windows that are displayed within a virtual machine appear side by side next to the windows of your host.Virtual Box Manual
We should not run seamless mode for a Ubuntu/Unity guest where the application menu was integrated in the top bar (Unity default) and will not be accessible in seamless mode.
To have access to the application's menu from seamless mode with Unity in the guest we may want to remove the packages responsible for the globalmenu:
sudo apt-get remove appmenu-*
sudo apt-get remove firefox-globalmenu # for Firefox