--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/browsers/firefox.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/browsers/firefox.py
@@ -299,22 +299,25 @@ class FirefoxBrowser(Browser):
 
         try:
             return self.runner.process_handler.pid
         except AttributeError:
             return None
 
     def on_output(self, line):
         """Write a line of output from the firefox process to the log"""
-        data = line.decode("utf8", "replace")
-        if self.stack_fixer:
-            data = self.stack_fixer(data)
-        self.logger.process_output(self.pid(),
-                                   data,
-                                   command=" ".join(self.runner.command))
+        if "GLib-GObject-CRITICAL" in line:
+            return
+        if line:
+            data = line.decode("utf8", "replace")
+            if self.stack_fixer:
+                data = self.stack_fixer(data)
+            self.logger.process_output(self.pid(),
+                                      data,
+                                      command=" ".join(self.runner.command))
 
     def is_alive(self):
         if self.runner:
             return self.runner.is_running()
         return False
 
     def cleanup(self):
         self.stop()