The following link may be useful in your situation: managing the flash plugin

I had that same problem and it drove me nuts .. here is what I did and it worked the first time I did it. Download Google Chrome. Use Chrome to go to the Adobe flash page and download it. When asked, choose ( for all other browsers) Then close it all out when you're done downloading it. Closing ALL browsers before installing it. When installed , shut down then restart the pc. Check to make sure its working in FireFox and if so, shut down and restart again just to be sure. Mine has been working ever since I did that. Its something in Firefox that isnt remembering the install. Download it from Chrome then do what I said above. Mine is working great now.