In discussions with Mozilla.org it may not be possible to get them to backport the necessary changes from Firefox 3 to make the new Java Plug-In run on top of Firefox 2.
As an alternate solution, we would like to be able to turn on the new Java Plug-In for Firefox 3, while continuing to use the old one for Firefox 2. In order to enable this functionality, changes on the browser side are needed. Mozilla bug 406040 has been filed, and a patch supplied: https://bugzilla.mozilla.org/show_bug.cgi?id=406040 . We are waiting for a response from Mozilla.org regarding the integration of this patch.
Once it is integrated, we will need to change the switching mechanism between the old and new plug-ins. We will need to stop moving files around on the local disk when toggling the switch. The only change will be the setting of the UseNewJavaPlugin registry key. The "unused_plugin" directory will be renamed to "new_plugin", and the only DLL that will be built into that directory will be npjp2.dll. Changes are necessary to the bootstrapping code of the new plug-in to handle the fact that npjp2.dll is being loaded out of a subdirectory of the JRE's bin directory. The installer will need to be modified to put a copy of msvcr71.dll alongside npjp2.dll in the new_plugin directory. The latter should be done as code in the installer rather than bundling a completely separate copy of this DLL into the bundles.