After having several issues with Firefox over the past couple months when installing new add-ons, I decided yesterday to uninstall Firefox and reinstall.  Happily I can report that my issues with installing new add-ons has been fixed.  However, I opened Flex Builder 3 this morning to work and after installing the debug version of Flash Player 10, I began to receive an error message when running or debugging my application.  The error reads:

Flash Player Not Found

Flex Builder cannot locate the required version of 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?

Thinking this was probably a glitch, I went ahead and ran the app.  No debug.  grrrr...

I uninstalled Flash Player, and restarted my machine, and performed a fresh install of Flash Player 10 debug.  Same result.  grrr...

I Googled the error and found that many have come across this error and the majority of the resolutions involved copying the file C:\WINDOWS\system32\Macromed\Flash\NPSWF32.dll into the Firefox plugins directory.  So I did this and sure enough, the error went away.  Of course, I wasn't satisfied with having an orphaned file just randomly floating around on my hard drive so I tried a different approach.  This now became an academic exercise.  So I deleted my orphaned file from the Firefox/plugins directory.

I went inside of Flex Builder 3 and going to Window > Preferences > General > Web Browser and noticed that Flex Builder was set to use the "Default system Web Browser" and instead I changed the value to "Firefox."  I ran my application in both normal and debug mode and the error was gone.  I checked that I was able to hit breakpoints and trace my code.  Everything worked perfectly.  

Hopefully this approach will help someone to not have to spend 30 minutes installing and reinstalling the Flash Player 10 debugger ending up with a manual copy of an orphaned file.  Give it a try and let me know if it works for you.

Cheers!