I open the forum specific search by clicking in the blue bar above on "Search" with three fingers in FireFox to open the forum specific advanced search window. You can also click the down arrow next to Search above and click on "
Click here to use the forum search instead" to open the forum search. In that window, at the bottom-left corner, see the heading "Sort results by." Set the two options to "Last Posting date" and "in descending order." Done.
Bob