--- a/dom/browser-element/mochitest/browserElementTestHelpers.js
+++ b/dom/browser-element/mochitest/browserElementTestHelpers.js
@@ -105,25 +105,31 @@ const browserElementTestHelpers = {
 //    dislike that.
 //
 //    Unfortunately setting network.disable.ipc.security to false before the
 //    child process(es) created by this test have shut down can cause us to
 //    assert and kill the child process.  That doesn't cause the tests to fail,
 //    but it's still scary looking.  So we just set the pref to true and never
 //    pop that value.  We'll rely on the tests which test IPC security to set
 //    it to false.
+//
+//  * security.mixed_content.block_active_content: false
+//
+//    Disable mixed active content blocking, so that tests can confirm that mixed
+//    content results in a broken security state.
 
 (function() {
   var oop = location.pathname.indexOf('_inproc_') == -1;
 
   browserElementTestHelpers.lockTestReady();
   SpecialPowers.setBoolPref("network.disable.ipc.security", true);
   SpecialPowers.pushPrefEnv({set: [["browser.pageThumbs.enabled", false],
                                    ["dom.ipc.browser_frames.oop_by_default", oop],
-                                   ["dom.ipc.tabs.disabled", false]]},
+                                   ["dom.ipc.tabs.disabled", false],
+                                   ["security.mixed_content.block_active_content", false]]},
                             browserElementTestHelpers.unlockTestReady.bind(browserElementTestHelpers));
 })();
 
 addEventListener('unload', function() {
   browserElementTestHelpers.removeAllTempPermissions();
 });
 
 // Wait for the load event before unlocking the test-ready event.