Created attachment 716046 [details] screenshot of extensions manager / plugin pane I took updates from Apple that included a new Java plugin. Because Java is evil and not to be trusted I loaded the add-ons pane for plugins after it updated and noticed that Java wasn't listed at all. I thought that was odd so I loaded about:plugins and noticed that it is listed there. It's also listed if I go to the plugin update tool, which means it's exposed to content.
Note that I didn't restart Firefox after doing the Java update underneath it. It's very possible that doing about:plugins refreshed the list of plugins, but the extension manager doesn't use the same method.
Possibly related to bug #398313?
Note that after a restart the Java plugin shows up in the extensions/add-on manager/plugin pane. So this looks like it's just a sync issue.
With it working properly after restart, this is bug 554780, which is fixed in Fx21.