The security updates revoke the certificates of authentication issued by a Turkish company that were recently found to have been compromised.
More immediately user-friendly is built-in support for the newest line of MacBook Pro laptops with Retina Display, or "HiDPI," a very high screen resolution that Apple says shrinks pixels down to a size at which humans can no longer see them. ("DPI" is an old printing term for "dots per inch.")
"If you are a Retina-owning Firefox user, upgrade to Firefox 18 now for the HiDPI text rendering alone," wrote the uncharacteristically exuberant Paul Ducklin on Sophos' Naked Security blog. "It really is groovy."