If you’re a Flex 2 developer you should download the newly released Flash player 9.0.r45 debug version.

Once installed the new player remember to apply also this patch to Flex builder 2, otherwise it will throw you an error every time debugging an app (expecially if you’re using Firefox).
This patch also does the following:

“This patch allows the Flex SDK and Flex Builder to use SWFs and SWCs created by Flash CS3 Professional. After applying the patch you should be able to take classes and symbols compiled with Flash CS3 Professional and compile them into a Flex application. You should also be able to take classes and libraries compiled with Flex and use them in a Flash CS3 Professional project.

In addition, this patch allows the Flex debugger (fdb) to function correctly with Flash Player 9 (9.0.45.0) in the Mozilla browser.”