If you upgraded to Firefox 3 you know that you can no longer use the Flash Switcher Plug-in. Here we have a neat little browser-independent widget for the Mac. It works well except installation is bit of a PITA. So far I have not been able to install Flash Player 8 and down because I can’t find one that’s for Intel-based Mac. I've already tried the Archived Flash Players on Adobe and had no luck (they are PowerPC-based). Let me know if you can find any previous Flash Player that works on Intel-based Mac. But it serves the purpose well for testing between FP9 or FP9 debug and FP10.
Update: The Flash Switcher Plug-in for Firefox has indeed been updated to support FF3. Thanks JohnO. It's still a great tool if you use FF and on a PC. But if you're on a Mac and don't want to be constrained by browser type the FlashSwitcher X is there for you to use.