Bug 1948258 added a way to hide the Extensions Button, but not a way to show it. This patch makes the following changes: - always show the Extensions button in Customization mode, even if the user wants to hide the button unconditionally. - add menu item to toggle the visibility of the Extensions Button. - the new menu item is not shown anywhere except on the Extensions Button in Customization mode. Differential Revision: https://phabricator.services.mozilla.com/D248158