You could try an earlier version of Flash. Also, you need to use a different plugin for Firefox than for Internet Explorer. You can get other Flash versions here:

http://www.adobe.com/products/flashplayer/distribution3.html

Can you open up your Real Player and tell me what version you have? I think Real Player 15.0.6 is supposed to fix this problem. There was a conflict between Real Player and Flash this whole summer that broke Flash videos. Real Player released a update that might help in your case.

If that doesn't help, I think a addon might be to blame here. So do this:
Firefox button > Help > Troubleshooting Information
(or Tools > Help > Troubleshooting Information)

Then click the "Copy all to clipboard" button then paste it here. It will tell us what addons you have installed and I can see which one might be causing problems.

Gosh!, You are right!. Once I uninstall the Real Player, Everything works good,

Thank you Noah_SUMO , Thank you so much, :)

Try this:

Disable Realplayer's Web Download & Recording to fix Firefox and Flash problems