The returned source may actually consist of multiple schema sources from different directories, depending on which directories were given in XDG_DATA_DIRS and GSETTINGS_SCHEMA_DIR. For this reason, all lookups performed against the default source should probably be done recursively.
In lookup, recurse through the returned schema sources from Gio.SettingsSchemaSource.get_default as suggested by https://people.gnome.org/~gcampagna/docs/Gio-2.0/Gio.SettingsSchemaSource.get_default.html
This fixes Gnome Shell gsettings schema detection.
Firefox fails with:
I fixed this locally by switching Gio SettingsSchemaSource lookup recurse parameter to True per: Gio SettingsSchemaSource get_default comment telling: