Symantec Corp’s shares fell as much as 14 percent on Friday, a day after the security software maker fired its chief executive, raising concerns about its turnaround efforts. At least five brokerages downgraded the stock, which has fallen nearly 17 percent in the past six months as the company, known for its Norton antivirus software, struggles with falling PC sales and a lackluster show on smartphone and tablets. Steve Bennett, who took over as CEO in July 2012, had launched a strategic review of the business after replacing company veteran Enrique Salem, who was fired after three years on the job. “The first year was terrific, the last six months have been terrible,” MKM Partners analyst Israel Hernandez said.