--- a/testing/web-platform/harness/wptrunner/browsers/firefox.py
+++ b/testing/web-platform/harness/wptrunner/browsers/firefox.py
@@ -1,14 +1,15 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import os
 import subprocess
+import sys
 
 import mozinfo
 from mozprocess import ProcessHandler
 from mozprofile import FirefoxProfile, Preferences
 from mozprofile.permissions import ServerLocations
 from mozrunner import FirefoxRunner
 from mozcrash import mozcrash
 
@@ -186,17 +187,18 @@ class FirefoxBrowser(Browser):
             env_var = "DYLD_LIBRARY_PATH"
         elif mozinfo.isUnix:
             env_var = "LD_LIBRARY_PATH"
         else:
             env_var = "PATH"
 
 
         env[env_var] = (os.path.pathsep.join([certutil_dir, env[env_var]])
-                        if env_var in env else certutil_dir)
+                        if env_var in env else certutil_dir).encode(
+                                sys.getfilesystemencoding() or 'utf-8', 'replace')
 
         def certutil(*args):
             cmd = [self.certutil_binary] + list(args)
             self.logger.process_output("certutil",
                                        subprocess.check_output(cmd,
                                                                env=env,
                                                                stderr=subprocess.STDOUT),
                                        " ".join(cmd))