searchyc.com already has a Firefox extension but you may have missed it because searchyc.com reached its transfer quota when it was announced. See http://news.ycombinator.com/item?id=177628
Also see http://news.ycombinator.com/item?id=198537 for a demo of this forum with a working search box and see http://news.ycombinator.com/item?id=197644 for the most recent dataset of posts.