Fix prefs getter call in wptrunner Firefox executor
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1452952
gecko-commit: 55b99481fab156381335b6887fca26b6a8f0c0b3
gecko-integration-branch: mozilla-inbound
gecko-reviewers: ato
diff --git a/tools/wptrunner/wptrunner/executors/executormarionette.py b/tools/wptrunner/wptrunner/executors/executormarionette.py
index 0b3be64..d06ee3c 100644
--- a/tools/wptrunner/wptrunner/executors/executormarionette.py
+++ b/tools/wptrunner/wptrunner/executors/executormarionette.py
@@ -403,7 +403,7 @@
                 self.prefs.set(name, value)
 
         for name, value in new_environment.get("prefs", {}).iteritems():
-            self.executor.original_pref_values[name] = self.get_pref(name)
+            self.executor.original_pref_values[name] = self.prefs.get(name)
             self.prefs.set(name, value)