This package has a build requirement 'gecko-devel' which is fulfilled by 'xulrunner-devel'. But in the process of building the browser plugins two tools named 'xpidl' and 'xpt_link' are necessary. They werde once part of 'gecko-devel' but are now replaced by other tools. I haven't found any package that has these tools all the way back to 5.0. Maybe the makers of CentOS can give a hint. After all this package was build for C5. Thanks Michael