In order to support all update release versions of Java in Internet Explorer, we register CLSIDs for approximately 30 update release versions for each JRE family during installation. In other words, we have CLSIDs in the registry corresponding to approximately 1.5.0_01...1.5.0_30. In the framework of the new Java Plug-In, where the new plug-in's DLL is registered for all of these CLSIDs but the plug-in itself is capable of running on an earlier JRE release, this means that an individual applet can select any of these JRE versions, the new plug-in will be used for the launch of the applet, and it will run the applet on top of the appropriate version of the JRE.

On the Firefox side, we have historically exported a jpi-version only for the current version of the Java Plug-In in the MIME type table for the plug-in. In the context of the old Java Plug-In, this is the correct behavior. However, in the context of the new Java Plug-In, it is not. We would need MIME type entries for all past and forseeable future update releases for each JRE family in the new plug-in's MIME type table in order to have it invoked for an applet requesting any of these particular update releases.

It's unclear whether we want to move in this direction, or encourage people to use the java_version parameter in the applet tag instead. Regardless, it is something needing more thought.