QuickTime can sometimes take over as the handler for Flash content, either from the installation or by actively selecting it as the default handler.

If you don't want to upgrade to the last version of QuickTime, these step by step instructions show how to disable QuickTime and revert to the previous Flash player again:

1. Close Firefox, and then start QuickTime (from the Windows Start menu)
2. From the Edit menu, select Preferences, and then QuickTime preferences.
3. In the window that pops up, select the Browser tab.
4. Click the MIME settings button.
5. Under the Miscellaneous category, make sure "Flash Media" is NOT checked .
6. Click OK, then restart your computer.

Flash should now be handled by the previous Flash player, e.g. Adobe Flash Player.

Based on information from Flash (mozillaZine KB)