Macos – How to add a custom keyboard shortcut for a nested menu item

This is a known issue in Firefox for Mac. You should read this bug report because the author embellished the impact of the issue with humor and Lord of the Rings references. Actual discussion of the core issues and potential fix are in a different bug report.

From Mac OSX Keyboard Commands:

Full Keyboard Access (see Chapter 3)

  control+F1

    Toggle Full Keyboard Access on/off

  control+F2 (or control+M)

    Focus keyboard control on menu bar

  control+F3 (or control+D)

    Focus keyboard control on Dock

  control+F4 (or control+W)

    Focus keyboard control on active Window or cycle to next window

  control+F5 (or control+T)

    Focus keyboard control on toolbar

  control+F6 (or control+U)

    Focus keyboard control on palette (utility window)

  control+F7

    In windows and dialogs, switch focus to text boxes/lists/controls

  arrow keys

    Navigate active item

  return, enter, or spacebar

    Select highlighted item

  return or enter

    Select default dialog control (OK, Yes, No, Save, Open, etc.)

  escape

    Cancel action, menu, or dialog

control+F1 to enable Full Keyboard Access (only needed once), then

control+F2 (or control+M) is the command you are looking for.