diff --git a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadmonitor.js b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadmonitor.js
index f61a601092b3..c57089aacd19 100644
--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadmonitor.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadmonitor.js
@@ -49,8 +49,19 @@ function test() {
   let panel = document.getElementById("download-monitor");
   waitForExplicitFinish();
 
+  let acceptDialog = 0;
+  let confirmCalls = 0;
+  function promptObserver(aSubject, aTopic, aData) {
+    let dialogWin = aSubject.QueryInterface(Ci.nsIDOMWindow);
+    confirmCalls++;
+    if (acceptDialog-- > 0)
+      dialogWin.document.documentElement.getButton("accept").click();
+  }
+
+  Services.obs.addObserver(promptObserver, "common-dialog-loaded", false);
+
   // Add a new download
-  addDownload(dm, {
+  let [file, persist] = addDownload(dm, {
     resultFileName: "pbtest-1",
     downloadName: "PB Test 1"
   });
@@ -63,16 +74,19 @@ function test() {
   ok(!panel.hidden, "The download panel should be successfully added initially");
 
   // Enter the private browsing mode
+  acceptDialog = 1;
   pb.privateBrowsingEnabled = true;
+  is(confirmCalls, 1, "One prompt was accepted");
+  ok(pb.privateBrowsingEnabled, "The private browsing transition was successful");
 
-  setTimeout(function () {
+  executeSoon(function () {
     ok(panel.hidden, "The download panel should be hidden when entering the private browsing mode");
 
     // Add a new download
-    let file = addDownload(dm, {
+    let [file2, persist2] = addDownload(dm, {
       resultFileName: "pbtest-2",
       downloadName: "PB Test 2"
-    }).targetFile;
+    });
 
     // Update the panel
     DownloadMonitorPanel.updateStatus();
@@ -81,9 +95,12 @@ function test() {
     ok(!panel.hidden, "The download panel should show up when a new download is added");
 
     // Exit the private browsing mode
+    acceptDialog = 1;
     pb.privateBrowsingEnabled = false;
+    is(confirmCalls, 2, "One prompt was accepted");
+    ok(!pb.privateBrowsingEnabled, "The private browsing transition was successful");
 
-    setTimeout(function () {
+    executeSoon(function () {
       ok(panel.hidden, "The download panel should be hidden when leaving the private browsing mode");
 
       // cleanup
@@ -98,9 +115,10 @@ function test() {
       if (file.exists())
         file.remove(false);
 
+      Services.obs.removeObserver(promptObserver, "common-dialog-loaded", false);
       finish();
-    }, 0);
-  }, 0);
+    });
+  });
 }
 
 /**
@@ -154,7 +172,7 @@ function addDownload(dm, aParams)
   persist.progressListener = dl.QueryInterface(Ci.nsIWebProgressListener);
   persist.saveURI(dl.source, null, null, null, null, dl.targetFile);
 
-  return dl;
+  return [dl.targetFile, persist];
 }
 
 function createURI(aObj) {
diff --git a/browser/components/privatebrowsing/test/browser/staller.sjs b/browser/components/privatebrowsing/test/browser/staller.sjs
index b2906d5598a3..7aab1714bf12 100644
--- a/browser/components/privatebrowsing/test/browser/staller.sjs
+++ b/browser/components/privatebrowsing/test/browser/staller.sjs
@@ -55,5 +55,6 @@ function handleRequest(request, response) {
   }
 
   response.setHeader("Content-Type", "text/plain", false);
+  response.setHeader("Accept-Ranges", "none", false);
   stall();
 }