I found there were 3 version of Shockwave Flash Player in my Firefox Add-ons: v11, v13 & v14 on my Windows 7 64 bit system.
Only the latest version is necessary and older versions may have vulnerabilities that are not desirable.
Follow the steps below to the letter to uninstall all versions of Flash Player (you may wish to do steps #3a, 3b & 4 before installing a new release).
1. Go to Uninstall Flash Player | Windows and download the Flash Player Uninstaller
2. Close all browsers
3a. Right click on the Flash Player Uninstaller and select "Run as Administrator" (I do this for ALL installs in Win 7)
3b. This step may not be necessary, but I did so to be certain all versions were uninstalled: Repeat #3a as many times as you have versions of Flash Player
4. RESTART YOUR COMPUTER - do not skip this step or you may end up with more than one version again!
5. If you've not previously done so, download the latest version of the latest Flash Player from Adobe - Install Adobe Flash Player(UNCHECK ANY UNDESIRABLE OPTIONS - AFAIAC, all optional downloads are undesirable)
6. Right click on the latest Flash Player Installer and select "Run as Administrator"
Open Firefox and go to Add-ons. Now you'll only have the latest version of Shockwave Flash Player and you're good to go.
Happy computing...