Rereading your question, I realize that you would prefer for the menu to remain in the top bar, but to be visible at all times. The instructions below remove the global menu and return the menus to the individual program windows.
A safe way to completely disable the global menu is by uninstalling the related packages:
To revert to global menus, install the packages again
sudo apt-get autoremove indicator-appmenu appmenu-gtk appmenu-gtk3 appmenu-qt
Although it will make no difference in your user experience, you could save small amounts of memory and an unperceptible gain in performance by disabling the "Global menu bar integration" plugins in Firefox and Thunderbird (just for the purists).
sudo apt-get install appmenu-gtk appmenu-gtk3 appmenu-qt indicator-appmenu