Macworld
"Like Google, another reason Mozilla cited for dumping Leopard was that Apple has also ended support. The last time Apple patched bugs in Leopard was November 2011, and its most recent security update, in May 2012, disabled older copies of Flash Player to stymie Flashback rather than fix specific security flaws. Nor has Apple maintained Safari on OS X 10.5. The final update was issued over a year ago."
