Firefox says that my GNU Flash Player is out of date.
The GNU Flash Player web site says I have the latest version, Feb 2012.
One of you is not telling the truth.
The GNU Flash Player isn't being developed anymore, you should switch to Adobe Flash 11.2
I have tried to follow the instructions for the 'new' flash and it still shows GNU Flash in the Plugin's.
Did you try to uninstall the Gnash plugin?
You can set the plugin.expose_full_path pref to true on the about:config page to see the full path of plugins on the aboutlugins page.
It is best not to leave that pref set to true as it exposes that full path to web servers via the navigator.plugins object, so reset that pref to false after you are done with the aboutlugins page.
See "Manually uninstalling a plugin":