MacOS – Zooming on Xcode on Mac

The following Applescript will trigger the zoom button (that´s button 2) in the frontmost application´s window. You could make this a service and define a global shortcut for it or use an application like quicksilver to call it.

tell application "System Events"
set frontApplication to (get name of every process whose frontmost is true) as string
tell process frontApplication
    click button 2 of window 1
end tell
end tell 

Firefox uses XUL files to define its user interface on all platforms. I just did a little poking in my Firefox.app 4.0.1 bundle:

• The XUL files are stored in Firefox.app/Contents/MacOS/omni.jar. (jar files can be unpacked with any Zip archive tool, or the jar command-line tool which takes options like tar.)
• The relevant XUL file may be chrome/browser/content/browser/baseMenuOverlay.xul. See also the referenced DTD files for the definitions of the entities like &zoomWindow.label;.

Since you're editing the files anyway, you might as well add a shortcut there rather than changing the name to use the system shortcut customizer.