Please read this if you use Flex Builder, or if you use fdb (the command-line debugger included with the free Flex SDK):
There is a new Flash player, 9.0r45. But remember: Most people out there in the world get the “release” version of the player, but developers need the deubgger version of it.
Get the debugger player here. (Save that link! It is surprisingly hard to find, and it comes in handy each time a new player comes out, because that’s where you go to get debugging-enabled players.)
Also, there is a patch for Flex Builder and for Flex, to make them work with this new player. If you are using Firefox on Windows and you don’t have this patch, you may get a warning message every time you try to debug. Get that patch here.