DT plugin for Firefox is disabled when new plug-in is enabled

During the fix for 6635585 (Modify plug-in switching mechanism to better support Firefox), we moved the new plug-in's Firefox DLL into bin/new_plugin and changed Firefox 3 to conditionally search that directory for plugins instead of the JRE's bin directory, if the registry key indicating to use the new Java Plug-In is set.

Unfortunately, this broke the finding of the DT plugin for Firefox (npdeploytk.dll) because this is only present in the "bin" directory.

To fix this, we need to modify the installer to make a copy of npdeploytk.dll from the bin/ directory to the bin/new_plugin/ directory, and to clean up this copy during uninstallation.