Before building the project the following must be done. 1. ..\..\bin\xpidl -m header -I..\..\idl nsISimplePlugin.idl This will generate nsISimplePlugin.h which is needed to successfuly compile the plugin 2. ..\..\bin\xpidl -m typelib -I..\..\idl nsISimplePlugin.idl This will create nsISimplePlugin.xpt file which should go to the Mozilla Components folder. Mechanism of accessing the service manager is implemented in Mozilla builds starting with 0.9.4. It will not work with Netscape 6.1.