It has recently come to my attention that flash movies will not play in the Firefox browser without the help of a external plug-in. But, there is hope for the future of Firefox and flash. Adobe and Mozilla have teamed together on the Tamarin project to help to
alleviate the strain on web/flash developers that this causes. They are working on integrating action script into the Firefox browser to produce a more robust, cross-platform browsing experience for Firefox users, they expect to complete the project in 2008. Feel free to follow the link I have provided below, for a more detailed explanation of The Tamarin project. Plus, I have included a link to the plug-in required by Firefox to view flash. Thank you.
Tamarin Project(mozilla.org)
Flash plug-in and installation instructions for Firefox