LSUIPresentationMode no longer works in Yosemite
You can run

lsappinfo setinfo -app Firefox ApplicationType=UIElement


but you have to run it every time you open Firefox, it removes the menu bar completely, and it removes Firefox from the Dock. Commands like lsappinfo setinfo -app Firefox PresentationMode=Suppressed or lsappinfo setinfo -app Firefox UIPresentationMode=4 didn't seem to work.

(You can make the full screen mode actually usable in iTerm 2 by unchecking "Use Lion-style Fullscreen windows" and setting "Profiles > Window > Style" to "Fullscreen". Skim, Sequential, Movist, and Emacs also have an option to use a custom implementation for full screen windows.)