diff --git a/testing/web-platform/tests/tools/wptrunner/wptrunner/wptcommandline.py b/testing/web-platform/tests/tools/wptrunner/wptrunner/wptcommandline.py
--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/wptcommandline.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/wptcommandline.py
@@ -500,10 +500,13 @@
             print >> sys.stderr, "certutil-binary argument missing or not a valid executable"
             sys.exit(1)
         kwargs["certutil_binary"] = path
 
     if kwargs['extra_prefs']:
+        # If a single pref is passed in as a string, make it a list
+        if type(kwargs['extra_prefs']) in (str, unicode):
+            kwargs['extra_prefs'] = [kwargs['extra_prefs']]
         missing = any('=' not in prefarg for prefarg in kwargs['extra_prefs'])
         if missing:
             print >> sys.stderr, "Preferences via --setpref must be in key=value format"
             sys.exit(1)
         kwargs['extra_prefs'] = [tuple(prefarg.split('=', 1)) for prefarg in