Close Firefox (and any other browsers).
Go to 'Add or Remove Programs' (or 'Programs and Features', depending on your version of Windows) from your Control Panel.
Locate and remove 'Adobe Flash Player Plug-in'.
Restart your computer.
Open Firefox.
Go here to download and install Flash Player again:
http://get2.adobe.com/flashplayer/
(NB: Before downloading, click to remove the tick where you're offered McAfee Security Scan Plus)
Read the instructions on the screen. (I seem to recall that Flash Player doesn't self-install; I think that, after the download has finished, you'll need to go to Tools > Downloads and double-click on the downloaded file to actually install it - but the on-screen instructions will tell you anyway).
Job done.