Completely disable "this plugin is vulnerable and should be updated" page in Firefox 18.0.2
Solution 1:
Solution discovered! Posting for anyone else who might need it as looks like the documentation is out of date.
- Open the about:config page by typing it into the address bar and accept the warning
- Search for the following option: Extensions.blocklist.enabled
- Toggle it to false by double clicking on it
This option is documented as a blacklist against known malicious extensions however it now also toggles the checking of plugins for vulnerabilities as well.
A warning: Turning this option to false lowers the security of your browser - not only will it stop checking for plugin vulnerabilities but Firefox will not prevent you from installing known malicious extensions that could hijack your browser or do any number of other nasty things.
The best solution when you see this message is to update the vulnerable plugin. I am only doing this because it is necessary and the kiosks where it will be used are heavily walled off from all but a handful of completely trusted sites.
Hope that saves someone else from all the searching I had to do!
Sources: https://support.mozilla.org/en-US/questions/949746 / http://kb.mozillazine.org/Extensions.blocklist.enabled