This add-on affect search queries coming from the search bar and Omnibar. By default Firefox will always search in a new tab when browser.search.openintab is true. This add-on makes Firefox search in new tab only when current tab is not a new tab page. It also makes Omnibar behave in the same manner.
To get Omnibar to open searches in new tab, make sure to turn on "Enable search for quires without keyword or operator"
Please make sure Omnibar is up-to-date to avoid potential issues.