diff --git a/testing/mochitest/mach_commands.py b/testing/mochitest/mach_commands.py
index 3de801a8f047..3aa7304ac02a 100644
--- a/testing/mochitest/mach_commands.py
+++ b/testing/mochitest/mach_commands.py
@@ -243,6 +243,7 @@ class MochitestRunner(MozbuildObject):
             start_at=None,
             end_at=None,
             e10s=False,
+            enable_cpow_warnings=False,
             strict_content_sandbox=False,
             nested_oop=False,
             dmd=False,
@@ -383,6 +384,7 @@ class MochitestRunner(MozbuildObject):
         options.startAt = start_at
         options.endAt = end_at
         options.e10s = e10s
+        options.enableCPOWWarnings = enable_cpow_warnings
         options.strictContentSandbox = strict_content_sandbox
         options.nested_oop = nested_oop
         options.dumpAboutMemoryAfterTest = dump_about_memory_after_test
@@ -623,6 +625,11 @@ def add_mochitest_general_args(parser):
         action='store_true',
         help='Run tests with electrolysis preferences and test filtering enabled.')
 
+    parser.add_argument(
+        '--enable-cpow-warnings',
+        action='store_true',
+        help='Run tests with unsafe CPOW usage warnings enabled.')
+
     parser.add_argument(
         '--strict-content-sandbox',
         action='store_true',
diff --git a/testing/mochitest/mochitest_options.py b/testing/mochitest/mochitest_options.py
index fb6d743a5924..b2318fcd0dd5 100644
--- a/testing/mochitest/mochitest_options.py
+++ b/testing/mochitest/mochitest_options.py
@@ -440,6 +440,11 @@ class MochitestOptions(ArgumentParser):
                   "used multiple times in which case the test must contain "
                   "at least one of the given tags.",
           }],
+        [["--enable-cpow-warnings"],
+         {"action": "store_true",
+          "dest": "enableCPOWWarnings",
+          "help": "enable logging of unsafe CPOW usage, which is disabled by default for tests",
+          }],
     ]
 
     def __init__(self, **kwargs):
diff --git a/testing/mochitest/runtests.py b/testing/mochitest/runtests.py
index dc183ae649b6..922c1f461d98 100644
--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -1519,6 +1519,11 @@ class Mochitest(MochitestUtilsMixin):
         if debugger and not options.slowscript:
             browserEnv["JS_DISABLE_SLOW_SCRIPT_SIGNALS"] = "1"
 
+        # For e10s, our tests default to suppressing the "unsafe CPOW usage"
+        # warnings that can plague test logs.
+        if not options.enableCPOWWarnings:
+            browserEnv["DISABLE_UNSAFE_CPOW_WARNINGS"] = "1"
+
         return browserEnv
 
     def cleanup(self, options):