In both, with my existing old build of Java, I got a placeholder image like this:
https://dl.dropboxusercontent.com/u/1643240/outdated_java.pn...
Clicking it took me to the update page. Exactly what you want. There was an option in the top-left corner to forcibly load it, which is fine - updating is the right move.
Once I updated and uninstalled the old JRE, in Firefox 24 the applet I was trying loaded silently without any confirmation. It was not blacklisted.
In Firefox Nightly, once Java is updated, I see this placeholder where the applet would have been:
https://dl.dropboxusercontent.com/u/1643240/activate_java.pn...
Clicking the placeholder opens a prompt asking if I want to allow the plugin once or allow it always on this site. Very straightforward.
Other than the fact that modern Java 7 is not blocked by default in Firefox 24 for me (maybe they didn't roll that out yet?), everything works fine here, and I don't see any catastrophic UI mistakes, developer/enterprise-hostile design, or attempts at destroying the web.