When starting MyFaces with the appropriate configurations in GlassFish's sun-web.xml, everything functions properly except for this ugly Exception at startup:
SCHWERWIEGEND: Cannot find private field _firstInstance from ExternalContext
This is caused because MyFaces provides this field in its implementation of javax.faces.context.ExternalContext, but Mojarra does not. And althought we set the right configuration parameters for using MyFaces, GlassFish wants to use Mojarra's version of this class when accessing the field via reflection.
The solution to this is very easy. We just have to store _firstInstance in a package private class with a specific name (_MyFacesExternalContextHelper) so we won't run into any classloading problems.