Cherryfizz,
I agree with my esteemed colleague winchester73, the first step that we should try is a complete uninstall of Flash followed by a re-install of the appropriate version.
Firstly, please go to the following link, carefully read the instructions (print them out if necessary) and download the uninstaller for Windows:
http://kb2.adobe.com/cps/141/tn_14157.htmlIf there is anything that you do not understand in the instructions on that page please seek advice here before proceeding. Once you are happy please follow the Adobe instructions to remove the application. When the process is complete reboot the machine, even if you are not prompted to do so.
Now we will get the correct version for Firefox. Go to the following link
http://get.adobe.com/flashplayer/otherversions/In step 1 use the drop down box to select the "Windows Vista/XP/2003/2000" version and click "Continue"
In step 2 select the second option "Flash Player 10 for Windows". You will see that under the entry "System Requirements" the browser entry changes to "Firefox/Safari/Opera"
Click on "Agree and install now". When all is complete try viewing a Youtube video and let us know what happens.