* FTSEurofirst 300 retreats from highs hit on Thursday* Telefonica falls after Q1 profits miss forecasts* Prospect of new ECB stimulus keeps some traders bullishBy Tricia WrightLONDON, May 9 (Reuters) - European shares retreated on Friday as a disappointing business update from leading telecoms group Telefonica TEF.MC sobered the mood after a rally the previous day.The pan-European FTSEurofirst 30