Glassfish runs its own version of JSF, so the version you're including with your project doesn't matter. If you wan't Glassfish to use it you'll have to upgrade it manually, the jars are under glassfish/modules.
Thanks, that solved the problem. I removed the old javax.faces.jar from glassfish/modules, and replaced it with javax.faces-2.2.6.jar. No more error.