Ever get tired of installing and de-installing Flash Players for testing? This free downloadable extension for Firefox by Sephiroth is the answer!
You can download the extension here. Then open Firefox, and go to Tools > Add Ons, and drag and drop the extension file in the window to install it. Then all you have to do is open the file that you want to test, and switch from Flash Player to Flash Player by using the button in the lower right hand corner.
If you really want to make sure, that your Flash file will run on Flash Player 7, with all the new features that Flash 8 offers, just save out your file as a Flash MX file. If you have anything that will not run for Flash Player 7, it will give you a warning. Save it, and publish it, and your final file should run fine on Flash Player 7.