I've done some research at mozilla and adobe and tests with both recent LTS-versions (14.04 and 12.04) and I'm pretty sure it is just a numbering problem of the flash-plugin!
To make a long story short:
If you open in firefox "tools" - "add-ons" - "plugins" respectively look at the mozilla check and you've got version 11,2,202,425 (notice the commas!), you will get this error message.
and if your version is 18.104.22.1685 (same version, just with dots at the numbering) everything is fine.
So don't panic, your system is safe! ;-)
But I can understand that this message is annoying and so this would be the easiest way to get rid of it:
open a terminal and insert
sudo apt-get update && sudo apt-get install -y flashplugin-installer, type your root-password (no characters will be displayed) and just wait till executed!
I would recommend this solution just in the case you have already installed the latest version but still get the error message because of the differences between the flash-packages!