These days I ran into a problem. I updated the flash player (from 9.0.28) to the last version (9.0.47 - moviestar) and after that the debug didn’t want to work anymore. The error that Flex Builder 2.0.1 displays is:

C:\Program Files\Mozilla Firefox\plugins\NPSWF32.dll

Flex Builder cannot locate the required version of the Flash Player. You may need to install Flash Player 9.0 or reinstall Flex Builder. Do you want to try to run your application with the current version?

I searched the internet but nothing helped me but the ultimate version was to backup all my projects and uninstall the whole Flex Builder 2.0.1 application together with the plugins installed to help me in development (like Subversion).

The very funny thing was that after finishing uninstall I noticed some folders left in Flex Builder 2 program file folder… and among those was Player Backup\debug where a lot of backups for problems with flash players. Next time after updating your flash player if you need to use the Flex Builder’s own flash player (which has debug) go to C:\Program Files\Adobe\Flex Builder 2\Player Backup\debug and run “Install Flash Player 9.exe” to get back to the old version.