Alessandro posted a cool extension for Firefox that allows a developer to swap out different versions of the Adobe Flash Player without installers or browser restarts. It’s a great tool for testing player compatibility, and you’ll be able to see instantly if your Flash version-checking script works correctly. Best of all, it comes with every player all the way back to Flash 2! How’s that for overkill? 
After I installed the extension, I immediately added the Flash 9 debug player to the list. Until now, I specifically ran the release player on Firefox and the debug player in IE. I didn’t want to limit my performance when checking out the cool AS3 experiments everyone has been making. Now, I can easily switch to debug mode to catch bugs when I’m working on my own cool experiments.
By the way, if you’re looking for something a little more arcane (I always a found it little difficult to use), you might want to check out Flash Plugin Switcher. It works for both IE and Firefox.
I actually prefer the old kewbee plugin switcher, as it’s easier to install new players. I keep on switching between r16 debug and r18 fullscreen, and neither is included. Maybe next version…
I didn’t think it was too hard to add a new version. You can install Flash normally, and then copy the DLL to the plugins directory for Flash Switcher. You can even change the location of this directory so that you don’t need to go searching for the default obscure location.