Flex Builder cannot locate the required version of the Flash Player. You might need to install Flash Player 9 or reinstall Flex Builder. Do you want to try to run your application with the current version?
Flex Builder cannot locate the required version of the Flash Player. You might need to install Flash Player 9 or reinstall Flex Builder. Do you want to try to run your application with the current version?
C:\Program Files\Mozilla Firefox\plugins\NPSWF32.dll
Flex Builder cannot locate the required version of the Flash Player. You might need to install Flash Player 9 or reinstall Flex Builder. Do you want to try to run your application with the current version?

I'm using Adobe Flex Builder 3.0. My default web browser was set to Firefox and went I tried to debug my code, I received the above error.
Solution for Firefox Browser:
-
Uninstalling the current version of Flash Player by using the Adobe Flash Player uninstaller
- In my case, I downloaded this file Download the Windows Flash Player 10 Plugin content debugger (for Netscape-compatible browsers) (EXE, 2.09 MB) from Adobe.com . After the installation, try to debug the Flex application again, the warning dialogue box should not appear.

I set my default browser to Internet Explorer and debug my Flex application, I received the above error.
Solution for Internet Explorer Browser:
-
Uninstalling the current version of Flash Player by using the Adobe Flash Player uninstaller (Skip this step if you already gone through Solution for Firefox Browser)
- Download the Windows Flash Player 10 ActiveX control content debugger (for IE) (EXE, 2.12 MB) from Adobe.com . After the installation, try to debug the Flex application again, the warning dialogue box should not appear.