use the right language
How not to do it:
> "your current version of Flash Player can cause security and stability issues" [...] "you should update Adobe Flash Player right now."
a bit better:
"Your current version of Flash Player can cause firefox to crash, and is unsafe because it can allow hackers to install their software on your computer which could allow them to collect details of anything you do online, including the websites you browse, the emails you send, your passwords, and any other personal information including details of your internet banking. This information may potentially allow hackers to impersonate you online. You can remove this security risk by updating Adobe Flash Player now, and keeping it up to date."
Should help. Aside from telling them to remove the bloody plugin altogether, which ain't going to happen.