diff --git a/layout/tools/reftest/runreftest.py b/layout/tools/reftest/runreftest.py
index 1880b4f9a1be..cd26d20551b4 100644
--- a/layout/tools/reftest/runreftest.py
+++ b/layout/tools/reftest/runreftest.py
@@ -557,13 +557,13 @@ class RefTest(object):
             stepOptions = copy.deepcopy(options)
             stepOptions.repeat = VERIFY_REPEAT
             stepOptions.runUntilFailure = True
-            stepOptions.environment.append("MOZ_CHAOSMODE=3")
+            stepOptions.environment.append("MOZ_CHAOSMODE=0xfb")
             result = self.runTests(tests, stepOptions)
             return result
 
         def step4():
             stepOptions = copy.deepcopy(options)
-            stepOptions.environment.append("MOZ_CHAOSMODE=3")
+            stepOptions.environment.append("MOZ_CHAOSMODE=0xfb")
             for i in range(VERIFY_REPEAT_SINGLE_BROWSER):
                 result = self.runTests(tests, stepOptions)
                 if result != 0:
diff --git a/testing/mochitest/runtests.py b/testing/mochitest/runtests.py
index f6b84516c028..445c19ac87c5 100644
--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -2558,7 +2558,7 @@ toolbar#nav-bar {
             stepOptions = copy.deepcopy(options)
             stepOptions.repeat = VERIFY_REPEAT
             stepOptions.keep_open = False
-            stepOptions.environment.append("MOZ_CHAOSMODE=3")
+            stepOptions.environment.append("MOZ_CHAOSMODE=0xfb")
             stepOptions.profilePath = None
             result = self.runTests(stepOptions)
             result = result or (-2 if self.countfail > 0 else 0)
@@ -2569,7 +2569,7 @@ toolbar#nav-bar {
             stepOptions = copy.deepcopy(options)
             stepOptions.repeat = 0
             stepOptions.keep_open = False
-            stepOptions.environment.append("MOZ_CHAOSMODE=3")
+            stepOptions.environment.append("MOZ_CHAOSMODE=0xfb")
             for i in xrange(VERIFY_REPEAT_SINGLE_BROWSER):
                 stepOptions.profilePath = None
                 result = self.runTests(stepOptions)
diff --git a/testing/web-platform/tests/tools/wptrunner/wptrunner/stability.py b/testing/web-platform/tests/tools/wptrunner/wptrunner/stability.py
index 605b40281a1b..d6befa802fda 100644
--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/stability.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/stability.py
@@ -340,7 +340,7 @@ def check_stability(logger, repeat_loop=10, repeat_restart=5, chaos_mode=True, m
                     output_results=True, **kwargs):
     kwargs_extras = [{}]
     if chaos_mode and kwargs["product"] == "firefox":
-        kwargs_extras.append({"chaos_mode_flags": 3})
+        kwargs_extras.append({"chaos_mode_flags": "0xfb"})
 
     steps = get_steps(logger, repeat_loop, repeat_restart, kwargs_extras)
 
diff --git a/testing/xpcshell/runxpcshelltests.py b/testing/xpcshell/runxpcshelltests.py
index f1ad5ef4706c..bb4b70ba446f 100755
--- a/testing/xpcshell/runxpcshelltests.py
+++ b/testing/xpcshell/runxpcshelltests.py
@@ -1697,7 +1697,7 @@ class XPCShellTests(object):
             def step2():
                 # Run tests sequentially, with MOZ_CHAOSMODE enabled.
                 sequential_tests = []
-                self.env["MOZ_CHAOSMODE"] = "3"
+                self.env["MOZ_CHAOSMODE"] = "0xfb"
                 for i in range(VERIFY_REPEAT):
                     self.testCount += 1
                     test = testClass(test_object, retry=False,