Netscape cannot find the Plugin Downloader Plugin. Without the Plugin Downloader Plugin, you cannot automatically download and install plugins. Please visit http://www.netscape.com/ to install the Plugin Downloader Plugin.
If you are receiving this message, then ensure that you are using the "NULL" plugin from Netscape - npnul32.dll. Using this plugin should eliminate the prompts asking for the "Plugin Downloader Plugin".
Add the following script to the USER.JS file to remove the plugin downloader popup screen:
user_pref("plugin.display_plugin_downloader_dialog", false);
In Edit | Preferences | Advanced | Software Installation
Disable: Software Installation
Disable: Check for Updates