It’s not take a long time, for Firefox user to get newest version of Firefox 3.6. It is only take a few day for Mozilla to release Firefox 3.6,15. Remember Mozilla has been released Firefox 3.6.14 and 3.5.17 few days ago. A new release shortly after a new version is usually indication of a security issue or a critical bug that needed fixing immediately. It is not clear why the new version has been pushed out.
Firefox 3.6 user will update automatically when they turn on automatic update. If not user can do automatic update from Help > Check for Update
Otherwise user can download Firefox 3.6.15 from Firefox website then install it as usual.