Bug 1628607 - Fix teardown in SingleInstanceManager, r=maja_zf

Differential Revision: https://phabricator.services.mozilla.com/D70331

--HG--
extra : moz-landing-system : lando

testing/web-platform/tests/tools/wptrunner/wptrunner/browsers/firefox.py

@@ -319,7 +319,7 @@ class SingleInstanceManager(FirefoxInstanceManager):
         return self.current
 
     def teardown(self, force=False):
-        for instance, skip_marionette in [self.previous, self.current]:
+        for instance in [self.previous, self.current]:
             if instance:
                 instance.stop(force)
                 instance.cleanup()
@@ -345,11 +345,14 @@ class PreloadInstanceManager(FirefoxInstanceManager):
         return self.current
 
     def teardown(self, force=False):
-        for instance, skip_marionette in [(self.previous, False), (self.current, False), (self.pending, True)]:
+        for instance, skip_marionette in [(self.previous, False),
+                                          (self.current, False),
+                                          (self.pending, True)]:
             if instance:
                 instance.stop(force, skip_marionette=skip_marionette)
                 instance.cleanup()
 
+
 class BrowserInstance(object):
     shutdown_timeout = 70