browserSettings.openBookmarksInNewTabs

A BrowserSetting object whose underlying value is a boolean.

If set to true, then when the user selects a bookmark, it will be opened in a new tab. If set to false (the default) bookmarks are opened in the current tab.

Set the setting to true:

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

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