In terms of why Firefox was the most exploited browser at the 2014 Pw2Own event, money likely plays a key role.
“Pwn2Own offers very large financial incentives to researchers to expose vulnerabilities, and that may have contributed in part to the researchers’ decision to wait until now to share their work and help protect Firefox users,” Stamm said. “Mozilla also offers financial rewards in our bug bounty program, and this program’s success has inspired other companies to follow suit.” –

via Pwn2Own 2014 Claims IE, Chrome, Safari and More Firefox Zero-Days.