Each website is rendered in a separate process with a sandbox, each with an empty chroot + process namespace + network namespace + tiny seccomp-bpf + syscall whitelist.
Chrome also has a stronger sandbox, pioneers better SSL, supports PIE on binaries, uses pepper - doesnt use native Flash plugins, the JIT compiler does randomization / encryption tricks to make it hard to heap spray exploit code. They have their own hardened memory allocator called PartitionAlloc.
Etc Etc.
Firefox also had more critical CVEs in 2014 than Chrome: http://pastebin.com/raw.php?i=2CRyJkmV
And reports of sandbox escapes are less common in Chrome for a reason.