diff --git a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_DownloadLastDirWithCPS.js b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_DownloadLastDirWithCPS.js
index 0f7d72fd2f8..bcd19b1923d 100644
--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_DownloadLastDirWithCPS.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_DownloadLastDirWithCPS.js
@@ -44,7 +44,7 @@ function setFile(downloadLastDir, aURI, aValue) {
 
 function clearHistoryAndWait() {
   clearHistory();
-  executeSoon(function() executeSoon(moveAlong));
+  executeSoon(() => executeSoon(moveAlong));
 }
 
 /*
@@ -73,7 +73,7 @@ function runTest() {
   registerCleanupFunction(function () {
     Services.prefs.clearUserPref("browser.download.lastDir.savePerSite");
     Services.prefs.clearUserPref("browser.download.lastDir");
-    [dir1, dir2, dir3].forEach(function(dir) dir.remove(true));
+    [dir1, dir2, dir3].forEach(dir => dir.remove(true));
     win.close();
     pbWin.close();
   });
diff --git a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadLastDir.js b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadLastDir.js
index 5402398b765..81b2943eeb8 100644
--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadLastDir.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadLastDir.js
@@ -33,7 +33,7 @@ function test() {
   // cleanup functions registration
   registerCleanupFunction(function () {
     Services.prefs.clearUserPref("browser.download.lastDir");
-    [dir1, dir2, dir3].forEach(function(dir) dir.remove(true));
+    [dir1, dir2, dir3].forEach(dir => dir.remove(true));
     MockFilePicker.cleanup();
   });
   prefs.setComplexValue("lastDir", Ci.nsIFile, tmpDir);
diff --git a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadLastDir_c.js b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadLastDir_c.js
index c5f36dd393f..5a04d1999df 100644
--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadLastDir_c.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadLastDir_c.js
@@ -28,13 +28,13 @@ function test() {
   // cleanup function registration
   registerCleanupFunction(function () {
     Services.prefs.clearUserPref("browser.download.lastDir");
-    [dir1, dir2, dir3].forEach(function(dir) dir.remove(true));
+    [dir1, dir2, dir3].forEach(dir => dir.remove(true));
     MockFilePicker.cleanup();
     validateFileName = validateFileNameToRestore;
   });
 
   // Overwrite validateFileName to validate everything
-  validateFileName = function(foo) foo;
+  validateFileName = foo => foo;
 
   let params = {
     fileInfo: new FileInfo("test.txt", "test.txt", "test", "txt", "http://mozilla.org/test.txt"),
diff --git a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_placesTitleNoUpdate.js b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_placesTitleNoUpdate.js
index 97c5c5ffcd9..32436b3cc71 100644
--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_placesTitleNoUpdate.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_placesTitleNoUpdate.js
@@ -51,7 +51,7 @@ add_task(function* test() {
     }]
   };
   PlacesUtils.asyncHistory.updatePlaces(place, {
-    handleError: function () ok(false, "Unexpected error in adding visit."),
+    handleError: () => ok(false, "Unexpected error in adding visit."),
     handleResult: function () { },
     handleCompletion: function () {}
   });
diff --git a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_sidebar.js b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_sidebar.js
index 2d89cdf3fc8..dbd74029d8c 100644
--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_sidebar.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_sidebar.js
@@ -44,44 +44,44 @@ function test() {
     return w;
   }
   function closeCachedWindows () {
-    windowCache.forEach(function(w) w.close());
+    windowCache.forEach(w => w.close());
   }
 
   // Part 1: NON PRIVATE WINDOW -> PRIVATE WINDOW
   openWindow(window, {}, 1).
     then(cacheWindow).
     then(openSidebar).
-    then(function(win) openWindow(win, { private: true })).
+    then(win => openWindow(win, { private: true })).
     then(cacheWindow).
     then(function({ document }) {
       let sidebarBox = document.getElementById("sidebar-box");
       is(sidebarBox.hidden, true, 'Opening a private window from reg window does not open the sidebar');
     }).
     // Part 2: NON PRIVATE WINDOW -> NON PRIVATE WINDOW
-    then(function() openWindow(window)).
+    then(() => openWindow(window)).
     then(cacheWindow).
     then(openSidebar).
-    then(function(win) openWindow(win)).
+    then(win => openWindow(win)).
     then(cacheWindow).
     then(function({ document }) {
       let sidebarBox = document.getElementById("sidebar-box");
       is(sidebarBox.hidden, false, 'Opening a reg window from reg window does open the sidebar');
     }).
     // Part 3: PRIVATE WINDOW -> NON PRIVATE WINDOW
-    then(function() openWindow(window, { private: true })).
+    then(() => openWindow(window, { private: true })).
     then(cacheWindow).
     then(openSidebar).
-    then(function(win) openWindow(win)).
+    then(win => openWindow(win)).
     then(cacheWindow).
     then(function({ document }) {
       let sidebarBox = document.getElementById("sidebar-box");
       is(sidebarBox.hidden, true, 'Opening a reg window from a private window does not open the sidebar');
     }).
     // Part 4: PRIVATE WINDOW -> PRIVATE WINDOW
-    then(function() openWindow(window, { private: true })).
+    then(() => openWindow(window, { private: true })).
     then(cacheWindow).
     then(openSidebar).
-    then(function(win) openWindow(win, { private: true })).
+    then(win => openWindow(win, { private: true })).
     then(cacheWindow).
     then(function({ document }) {
       let sidebarBox = document.getElementById("sidebar-box");
diff --git a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_theming.js b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_theming.js
index 432f47748e0..e2b8593d664 100644
--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_theming.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_theming.js
@@ -11,7 +11,7 @@ function testOnWindow(options, callback) {
   win.addEventListener("load", function onLoad() {
     win.removeEventListener("load", onLoad, false);
     windowsToClose.push(win);
-    executeSoon(function() callback(win));
+    executeSoon(() => callback(win));
   }, false);
 }
 
diff --git a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_ui.js b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_ui.js
index adb67d433a6..cbd2c60f88c 100644
--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_ui.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_ui.js
@@ -58,7 +58,7 @@ function test() {
       // execute should only be called when need, like when you are opening
       // web pages on the test. If calling executeSoon() is not necesary, then
       // call whenNewWindowLoaded() instead of testOnWindow() on your test.
-      executeSoon(function() aCallback(aWin));
+      executeSoon(() => aCallback(aWin));
     });
   };
 
diff --git a/browser/components/privatebrowsing/test/browser/head.js b/browser/components/privatebrowsing/test/browser/head.js
index a40101b7d45..c822ba8d1aa 100644
--- a/browser/components/privatebrowsing/test/browser/head.js
+++ b/browser/components/privatebrowsing/test/browser/head.js
@@ -57,7 +57,7 @@ function clearHistory() {
 function _initTest() {
   // Don't use about:home as the homepage for new windows
   Services.prefs.setIntPref("browser.startup.page", 0);
-  registerCleanupFunction(function() Services.prefs.clearUserPref("browser.startup.page"));
+  registerCleanupFunction(() => Services.prefs.clearUserPref("browser.startup.page"));
 }
 
 _initTest();