diff --git a/build/pgo/profileserver.py b/build/pgo/profileserver.py
index 6eab7889b0b9..aae5e7997dad 100755
--- a/build/pgo/profileserver.py
+++ b/build/pgo/profileserver.py
@@ -17,7 +17,6 @@ from mozhttpd import MozHttpd
 from mozprofile import FirefoxProfile, Preferences
 from mozprofile.permissions import ServerLocations
 from mozrunner import CLI, FirefoxRunner
-from six import string_types
 
 PORT = 8888
 
@@ -120,7 +119,7 @@ if __name__ == "__main__":
         interpolation = {"server": "%s:%d" % httpd.httpd.server_address}
         sp3_interpolation = {"server": "%s:%d" % sp3_httpd.httpd.server_address}
         for k, v in prefs.items():
-            if isinstance(v, string_types):
+            if isinstance(v, str):
                 v = v.format(**interpolation)
             prefs[k] = Preferences.cast(v)
 
diff --git a/build/upload_generated_sources.py b/build/upload_generated_sources.py
index 3e4b4acd599b..d635ac7c2d3c 100644
--- a/build/upload_generated_sources.py
+++ b/build/upload_generated_sources.py
@@ -12,6 +12,7 @@ import sys
 import tarfile
 import time
 from contextlib import contextmanager
+from queue import Queue
 from threading import Event, Thread
 
 import requests
@@ -20,7 +21,6 @@ from mozbuild.generated_sources import (
     get_s3_region_and_bucket,
 )
 from requests.packages.urllib3.util.retry import Retry
-from six.moves.queue import Queue
 
 # Arbitrary, should probably measure this.
 NUM_WORKER_THREADS = 10
diff --git a/build/valgrind/mach_commands.py b/build/valgrind/mach_commands.py
index 047e489e00fa..e48d5af31135 100644
--- a/build/valgrind/mach_commands.py
+++ b/build/valgrind/mach_commands.py
@@ -45,7 +45,6 @@ def valgrind_test(command_context, suppressions):
     from mozprofile.permissions import ServerLocations
     from mozrunner import FirefoxRunner
     from mozrunner.utils import findInPath
-    from six import string_types
     from valgrind.output_handler import OutputHandler
 
     build_dir = os.path.join(command_context.topsrcdir, "build")
@@ -75,7 +74,7 @@ def valgrind_test(command_context, suppressions):
             "server": "%s:%d" % httpd.httpd.server_address,
         }
         for k, v in prefs.items():
-            if isinstance(v, string_types):
+            if isinstance(v, str):
                 v = v.format(**interpolation)
             prefs[k] = Preferences.cast(v)