I am unable to upgrade the latest plugin for Flash Player

Yesterday I was asked to upgrade my Flash Player plugin, I followed the instructions and found that there was a problem - the new upgraded plugin wouldn't work.  I did everything I knew to change this - I uninstalled everything, I removed cache and cookie history - I disabled AVG - I reinstalled the latest version of Flash Player plugin, but nothing worked.  I have downloaded an older version of Flash Player and that is working but each time I have to give Firefox permission to allow an out of date plugin.  I don't know where the problem lies.  Prior to yesterday the plugin worked very well indeed.  I don't know if the trouble likes with Firefox - I wondered if their software was compatible to the new plugin update.

Please can you help me - I use Youtube video a lot and other videos and would find it very difficult if I am unable to access them because of this problem.

I use the latest version of Firefox, and my computer is an Apple Mac Book, running OS X 7.5