diff --git a/devtools/server/tests/mochitest/test_inspector-inactive-property-helper.html b/devtools/server/tests/mochitest/test_inspector-inactive-property-helper.html
index 2f8d2fb66648..8df04fb40968 100644
--- a/devtools/server/tests/mochitest/test_inspector-inactive-property-helper.html
+++ b/devtools/server/tests/mochitest/test_inspector-inactive-property-helper.html
@@ -11,10 +11,17 @@ SimpleTest.waitForExplicitFinish();
 
 (async function() {
   const { require } = ChromeUtils.import("resource://devtools/shared/Loader.jsm");
+  const Services = require("Services");
   const { inactivePropertyHelper } = require("devtools/server/actors/utils/inactive-property-helper");
   let { isPropertyUsed } = inactivePropertyHelper;
   isPropertyUsed = isPropertyUsed.bind(inactivePropertyHelper);
 
+  const INACTIVE_CSS_PREF = "devtools.inspector.inactive.css.enabled";
+  Services.prefs.setBoolPref(INACTIVE_CSS_PREF, true);
+  SimpleTest.registerCleanupFunction(() => {
+    Services.prefs.clearUserPref(INACTIVE_CSS_PREF);
+  });
+
   const FOLDER = "./inactive-property-helper";
 
   // Each file should `export default` an array of objects, representing each test case.