browserSettings.newTabPageOverride

A BrowserSetting object that can be used to get a string representing the URL for the "new tab" page: that is, the page that's loaded when the user opens a new empty tab.

Note that this is a read-only setting.

Get the current value of the new tab URL:

browser.browserSettings.newTabPageOverride.get({}).then(result => {
  console.log(result.value);
});