Comment by guns

  # Use 17 for Ctrl, 18 for Alt, 91 or 224 for Meta, 0 for none.
  - name: ui.key.accelKey
    type: uint32_t
  #ifdef XP_MACOSX
    value: 224
  #else
    value: 17
  #endif

  // user.js
  user_pref("ui.key.accelKey", 91);