Download the Flash Player uninstaller.
Save it - DO NOT run it yet.
Download the Adobe Flash Player installer directly by right clicking the following link and selecting "Save Link as".
Save the installer to your downloads folder - DO NOT run it yet.
Close your browser completely (check the Task Manager to be sure the Firefox process is no longer running).
Run the uninstaller to completely remove Flash Player.
Reboot your system.
BEFORE you run anything else, locate and run the installer to replace Flash Player.