browserSettings.openSearchResultsInNewTabs

A BrowserSetting object whose underlying value is a boolean.

If set to true, then when the user selects a term in the browser's search box, the search results are displayed in a new tab. If set to false (the default) the search results are shown in the current tab.

Note that this does not affect the behavior when selecting items from the omnibox/awesomebar,only the dedicated search box.

function logResult(result) {
  console.log(`Setting was modified: ${result}`);
}

browser.browserSettings.openSearchResultsInNewTabs
  .set({ value: true })
  .then(logResult);