In the latest version of Safari, Apple enhanced the browser with a security measure that ensures users make use of the latest version of Flash player. The browser will check for the latest version of Flash, and if your installed version is out of date, then it will disable the plug-in and prompt you to download the latest version, but this might not always be desired.
This security feature has been applauded by Adobe's Brad Arkin, who in a blog posting stated:
"We welcome today's initiative by Apple to encourage Mac users to stay up-to-date: With the Apple Safari 5.1.7 update released today, Apple is disabling older versions of Flash Player (specifically Flash Player 10.1.102.64 and earlier) and directing users to the Flash Player Download Center, from where they can install the latest, most secure version of Flash Player."
While this development is excellent news from a security standpoint and helps Apple regain some face after recent security mishaps on its part, some people may still wish to use older versions of Flash player for compatibility reasons. Even though new versions of the software will contain fixes to vulnerabilities, sometimes bugs may appear that result in broken functionality or poor performance, and having this auto-disable feature will force the use of faulty updates.
Luckily Apple's browser does not prevent the use of older versions of Flash, and merely disables them when newer versions are available. Therefore, if you need to revert back to a prior version of Flash when using Safari 5.1.7 or higher, you can re-enable the plug-in by going to the Macintosh HD/Library/Internet Plug-Ins (Disabled)/ folder and moving the "Flash Player.plugin" file from this folder into the Macintosh HD/Library/Internet Plug-Ins/ folder (the one that does not have "Disabled" in its name).
After you have moved the plug-in to the active plug-ins directory, then relaunch your Web browsers and you should be good to go. Safari should only check once for the latest version of Flash once and not continue to disable the plug-in every time you use the browser.