--- a/testing/mozharness/configs/mediatests/buildbot_posix_config.py
+++ b/testing/mozharness/configs/mediatests/buildbot_posix_config.py
@@ -38,13 +38,13 @@ config = {
     "download_symbols": "ondemand",
 
     "suite_definitions": {
         "media-tests": {
             "options": [],
         },
         "media-youtube-tests": {
             "options": [
-                "%(test_manifest)s"
+                "{test_manifest}"
             ],
         },
     },
 }

--- a/testing/mozharness/configs/mediatests/buildbot_windows_config.py
+++ b/testing/mozharness/configs/mediatests/buildbot_windows_config.py
@@ -9,18 +9,18 @@ external_tools_path = os.path.join(
 
 config = {
     "virtualenv_python_dll": 'c:/mozilla-build/python27/python27.dll',
     "virtualenv_path": 'venv',
     "exes": {
         'python': 'c:/mozilla-build/python27/python',
         'virtualenv': ['c:/mozilla-build/python27/python', 'c:/mozilla-build/buildbotve/virtualenv.py'],
         'hg': 'c:/mozilla-build/hg/hg',
-        'mozinstall': ['%s/build/venv/scripts/python' % os.getcwd(),
-                       '%s/build/venv/scripts/mozinstall-script.py' % os.getcwd()],
+        'mozinstall': ['{}/build/venv/scripts/python'.format(os.getcwd()),
+                       '{}/build/venv/scripts/mozinstall-script.py'.format(os.getcwd())],
         'tooltool.py': [sys.executable, 'C:/mozilla-build/tooltool.py'],
     },
 
     "find_links": [
         "http://pypi.pvt.build.mozilla.org/pub",
         "http://pypi.pub.build.mozilla.org/pub",
     ],
     "pip_index": False,
@@ -44,13 +44,13 @@ config = {
     "download_symbols": "ondemand",
 
     "suite_definitions": {
         "media-tests": {
             "options": [],
         },
         "media-youtube-tests": {
             "options": [
-                "%(test_manifest)s"
+                "{test_manifest}"
             ],
         },
     },
 }

--- a/testing/mozharness/configs/mediatests/jenkins_config.py
+++ b/testing/mozharness/configs/mediatests/jenkins_config.py
@@ -26,17 +26,17 @@ config = {
     'test_suite': 'media-tests',
 
     'suite_definitions': {
         'media-tests': {
             'options': [],
         },
         'media-youtube-tests': {
             'options': [
-                '%(test_manifest)s'
+                '{test_manifest}'
             ],
         },
     },
 
     'default_actions': [
         'clobber',
         'download-and-extract',
         'create-virtualenv',

--- a/testing/mozharness/configs/mediatests/taskcluster_posix_config.py
+++ b/testing/mozharness/configs/mediatests/taskcluster_posix_config.py
@@ -35,13 +35,13 @@ config = {
     ],
 
     "suite_definitions": {
         "media-tests": {
             "options": [],
         },
         "media-youtube-tests": {
             "options": [
-                "%(test_manifest)s"
+                "{test_manifest}"
             ],
         },
     },
 }

--- a/testing/mozharness/configs/mediatests/taskcluster_windows_config.py
+++ b/testing/mozharness/configs/mediatests/taskcluster_windows_config.py
@@ -38,13 +38,13 @@ config = {
     ],
 
     "suite_definitions": {
         "media-tests": {
             "options": [],
         },
         "media-youtube-tests": {
             "options": [
-                "%(test_manifest)s"
+                "{test_manifest}"
             ],
         },
     },
 }

--- a/testing/mozharness/mozharness/mozilla/testing/firefox_media_tests.py
+++ b/testing/mozharness/mozharness/mozilla/testing/firefox_media_tests.py
@@ -219,28 +219,28 @@ class FirefoxMediaTestsBase(TestingMixin
             cmd.append('--disable-e10s')
         if self.browsermob_script:
             cmd += ['--browsermob-script', self.browsermob_script]
         if self.browsermob_port:
             cmd += ['--browsermob-port', self.browsermob_port]
 
         test_suite = self.config.get('test_suite')
         if test_suite not in self.config["suite_definitions"]:
-            self.fatal("%s is not defined in the config!" % test_suite)
+            self.fatal("{} is not defined in the config!".format(test_suite))
 
         test_manifest = None if test_suite != 'media-youtube-tests' else \
             os.path.join(dirs['external-media-tests'],
                          'external_media_tests',
                          'playback', 'youtube', 'manifest.ini')
         config_fmt_args = {
             'test_manifest': test_manifest,
         }
 
         for s in self.config["suite_definitions"][test_suite]["options"]:
-            cmd.append(s % config_fmt_args)
+            cmd.append(s.format(**config_fmt_args))
 
         return cmd
 
     def query_minidump_stackwalk(self):
         """We don't have an extracted test package available to get the manifest file.
 
         So we have to explicitely download the latest version of the manifest from the
         mozilla-central repository and feed it into the query_minidump_stackwalk() method.
@@ -255,17 +255,17 @@ class FirefoxMediaTestsBase(TestingMixin
             url_base = 'https://hg.mozilla.org/mozilla-central/raw-file/default/testing/'
 
             dirs = self.query_abs_dirs()
             manifest_path = os.path.join(dirs['abs_work_dir'], 'releng.manifest')
             try:
                 self.download_file(urlparse.urljoin(url_base, tooltool_manifest),
                                    manifest_path)
             except Exception as e:
-                self.fatal('Download of tooltool manifest file failed: %s' % e.message)
+                self.fatal('Download of tooltool manifest file failed: {}'.format(e.message))
 
         return super(FirefoxMediaTestsBase, self).query_minidump_stackwalk(manifest=manifest_path)
 
     def run_media_tests(self):
         cmd = self._query_cmd()
         self.job_result_parser = JobResultParser(
             config=self.config,
             log_obj=self.log_obj,