By Alexei Oreskovic SAN FRANCISCO (Reuters) – Yahoo Incs online ad prices slid again in the fourth quarter and Alibaba, the Chinese e-commerce giant in which it owns a big stake, saw revenue growth decelerate from its recent rip-roaring pace. Yahoos overall revenue fell 6 percent in the last three months of the year to $1.266 billion, marking four consecutive quarters of eroding revenue. The core business is shrinking. Yahoos efforts to revamp its slumping business have come to the forefront following Chief Executive Officer Marissa Mayers decision to fire Chief Operating Officer Henrique de Castro this month, after only slightly more than a year on the job. The move marked the first major change of plans since Mayer took the helm in July 2012, and underscored Yahoos ongoing challenge to rekindle revenue growth.