A little over a month ago, Mozilla has announced that Firefox will be preventing all plug-ins except the current version of Flash from loading automatically, requiring users who explicitly want to use them to start them themselves.
Last week, Apple has decided to do the same with older versions of the Adobe Flash Player plug-in.
Users who attempt to view Flash content in Safari with an outdated plugin will be seeing the "Blocked Plug-in" notification. Clicking on it will show them the reason:
In order to view the content, they will be forced to update it.
The change was announced by Apple in a security note that stated that "to help protect users from a recent vulnerability, Apple has updated the web plug-in-blocking mechanism to disable older versions of the web plug-in: Adobe Flash Player."
There was no explanation as to which vulnerability it is, but it's probably one of the three fixed with the latest Flash Player update released two days earlier.