--- a/toolkit/content/tests/chrome/test_mousescroll.xul
+++ b/toolkit/content/tests/chrome/test_mousescroll.xul
@@ -74,17 +74,17 @@ https://bugzilla.mozilla.org/show_bug.cg
   </arrowscrollbox>
 
   <!-- test code goes here -->
   <script type="application/javascript"><![CDATA[
 
 /** Test for Bug 378028 **/
 /*   and for Bug 350471 **/
 SimpleTest.waitForExplicitFinish();
-SimpleTest.waitForFocus(runTests);
+SimpleTest.waitForFocus(prepareRunningTests);
 
 const IS_MAC = navigator.platform.indexOf("Mac") == 0;
 
 // Some tests need to wait until stopping scroll completely.  At this time,
 // setTimeout() will retry to check up to MAX_RETRY_COUNT times.
 const MAX_RETRY_COUNT = 5;
 
 const deltaModes = [
@@ -338,16 +338,31 @@ function runTests()
   var ret = gTestContinuation.next();
   if (ret.done) {
     var winUtils = SpecialPowers.getDOMWindowUtils(window);
     winUtils.restoreNormalRefresh();
     SimpleTest.finish();
   }
 }
 
+async function prepareRunningTests()
+{
+  // Before actually running tests, we disable auto-dir scrolling, becasue the
+  // horizontal scrolling tests in this file are mostly meant to ensure that the
+  // tested controls in the default style should only have one scrollbar and it
+  // must always be in the block-flow direction so they are not really meant to
+  // test default actions for wheel events, so we simply disabled auto-dir
+  // scrolling, which are well tested in
+  // dom/events/test/window_wheel_default_action.html.
+  await SpecialPowers.pushPrefEnv({"set": [["mousewheel.autodir.enabled",
+                                            false]]});
+
+  runTests();
+}
+
 function* testBody()
 {
   yield* testRichListbox("richlistbox");
   yield* testListbox("listbox");
   yield* testArrowScrollbox("hscrollbox");
   yield* testArrowScrollbox("vscrollbox");
 }
 

--- a/widget/tests/window_wheeltransaction.xul
+++ b/widget/tests/window_wheeltransaction.xul
@@ -167,18 +167,26 @@ gRootView.addEventListener("MozMouseScro
 gRootView.addEventListener("MozMouseScrollTransactionTimeout",
                            onTransactionTimeout, false);
 
 function finish()
 {
   window.close();
 }
 
-function onload()
+async function onload()
 {
+  // Before actually running tests, we disable auto-dir scrolling, becasue the
+  // tests in this file are meant to test scrolling transactions, not meant to
+  // test default actions for wheel events, so we simply disabled auto-dir
+  // scrolling, which are well tested in
+  // dom/events/test/window_wheel_default_action.html.
+  await SpecialPowers.pushPrefEnv({"set": [["mousewheel.autodir.enabled",
+                                            false]]});
+
   runNextTestList();
 }
 
 function onunload()
 {
   resetTimeoutPrefs();
   gPrefSvc.clearUserPref(kPrefSmoothScroll);
   gPrefSvc.clearUserPref(kPrefTestEventsAsyncEnabled);