GORE FIRES BACK, HOPING TO DAMPEN CONVENTION SPIRIT
Vice Pres Al Gore roars back into presidential campaign after a week's vacation, giving speech to International Association of Fire Fighters in Chicago in which defends himself against the barbs hurled at him at National Republican Convention and delivers a few of his own; accuses Republicans of running a showy, but insubstantial, convention; claims credit for helping create the nation's roaring economy when he cast a tie-breaking vote in Senate for president's 1993 economic stimulus package; photo (M)