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.
Interactive Section 
You saved my life too.. I couldn’t do it without your posting… Thanks a lot.
Thank You!!! 🙂
Hi guy.
I had the same problem and I did the same tips, but didn’t work. However, I solve my problem doing something a little different.
Go to Window > Preferences > General > Web Browser. Click on New button, add your favourite web browser (I use Firefox) and click Ok. Select the new item and click on Apply button (or Ok button in the same window). Try it! Works fine for me.
Thanks in advance!
Cadu is right…. I tried many solutions without success… an finally I did what he mentioned above… it works for me too…
Cadu’s solution also worked for me.
I already had firefox in the list of browsers, I just had to select it
I have tried to use your steps but it couldnt work.but i tried the one one this link and it worked http://www.ntaka.co.za/flash-cannot-locate-the-required-debug-player