OK. I just spent quite a while solving a problem I had. Hope the following helps if you encounter similar problem later.
What happened was I accidentally overwrote the debugger version of Flash Player for Firefox, and Flex Builder kept warning me that it couldn’t find the required version of Flash player no matter how many times I had reinstalled the right version. It didn’t work even when I was willing to switch over to IE for debugging purposes.
This link is from Adobe and is supposed to be helpful.
http://kb.adobe.com/selfservice/viewContent.do?externalId=758bf58b&sliceId=2
But this post was what really saved my life:
http://www.actionscript.org/forums/showthread.php3?t=145749
The key solution lies here:
Firefox was looking for the npswf32.dll in the ProgramFiles/Mozilla Fireforx/plugins directory yet Macromedia installed it in the window/system32/macromed/flash directory.
I copied it over to the Firefox directory and no more annoying warning.
