Bug 1452952 - Fix prefs getter call in wptrunner Firefox executor, r=ato

MozReview-Commit-ID: BEU8bpH4CVg

testing/web-platform/tests/tools/wptrunner/wptrunner/executors/executormarionette.py

@@ -403,7 +403,7 @@ class MarionetteProtocol(Protocol):
                 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)