There's a tram / train line to Liberec from Jablonec nad Nisou. It only takes a little time to arrive in Liberec. The journey is a also very cheap! During my last trip we had to change from tram to bus (labeled with the same number) that finally took us to Liberec but I don't know if this still exists.
Moving around in Jablonec is easy by walking cause the distances aren't too long and streets are normal.