The em:hidden property in install.rdf is a bad practice, and it is already being obsoleted: https://developer.mozilla.org/en/Install_Manifests#hidden. Add-on authors should be encouraged not to use it anymore.
Just added support for this to the new validator. I'll be pushing this update to github later this morning with some other changes.
This isn't really critical, so you can take this one as part of the new validator. Thanks.