Before upgrading to Windows 7 download the Windows 7 Upgrade Advisor Tool.  This tool will scan your computer's hardware, devices, and applications  for compatibility issues.  Once the scan is complete the Advisor will generate a report with guidelines on how to resolve potential issues. It will also provide recommendations on what to do prior to upgrading to Windows 7.  Click .    on this link to start the download Advisor Tool:
http://www.microsoft.com/downloads/details.aspx?FamilyID=1b544e90-7659-4bd9-9e51-2497c146af15&displaylang=en

You might also want to visit Microsoft Windows 7 Compatibility Center at:
http://www.microsoft.com/windows/compatibility/windows-7/en-us/default.aspx

Close the Firefox Browser and using Windows Internet Explorer reinstall the Firefox Windows Media Player Add-on. Here's the link to download the add-on:
http://port25.technet.com/pages/windows-media-player-firefox-plugin-download.aspx
For all other Windows Media Player Browser Plugin's:
http://www.microsoft.com/windows/windowsmedia/download/alldownloads.aspx