If you code in the Adobe Flash environment, the Flash Switcher extension is absolutely the best bet for backward compatibility for your Rich Internet Applications (RIA).
Installation was a little difficult. For novices, this may present a problem, because you need to download the extension and open it through Firefox. The installation isn't as easy as some.
When installed, the extension appears at the bottom of the Firefox in the status bar as the Flash logo. Click on the logo and it displays a list of Flash Player or Shockwave versions. Select the version you want to test your Flash or Shockwave RIA with and you are on your way.
When I started downloading the extension, I looked at the size. Woof! Every version of the Flash Player, from version 2.0 all the way up to version 9, is located in the Firefox extension. That's the whole reason the extension is 3 megs. I think it's worth it if you develop with Flash.
What's that? You can't find your version of Flash Player? Add it yourself. If you read the mini-faq on the site, you'll see how easy it is to add custom Flash Players or Shockwave Players to the list. I really like this feature. This extends the longevity of the FlashSwitcher extension.
There is also an option to remove the Flash Player entirely for testing purposes.
Aside from the installation issue, this extension is simple and provides a great solution to developers of RIA using Flash.