function toggleAllowFtp() {

  function toggle(current) {
    console.log(`Current value: ${current.value}`);
    browser.browserSettings.ftpProtocolEnabled.set({value: !current.value});
  }

  browser.browserSettings.ftpProtocolEnabled.get({}).then(toggle);
}

browser.browserAction.onClicked.addListener(() => {
  toggleAllowFtp();
});