--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -4402,8 +4402,35 @@ nsTranslationNodeList::GetLength(uint32_
 NS_IMETHODIMP
 nsDOMWindowUtils::WrCapture()
 {
   if (WebRenderBridgeChild* wrbc = GetWebRenderBridge()) {
     wrbc->Capture();
   }
   return NS_OK;
 }
+
+NS_IMETHODIMP
+nsDOMWindowUtils::SetSystemFont(const nsACString& aFontName)
+{
+  nsIWidget* widget = GetWidget();
+  if (!widget) {
+    return NS_OK;
+  }
+
+  nsAutoCString fontName(aFontName);
+  return widget->SetSystemFont(fontName);
+}
+
+NS_IMETHODIMP
+nsDOMWindowUtils::GetSystemFont(nsACString& aFontName)
+{
+  nsIWidget* widget = GetWidget();
+  if (!widget) {
+    return NS_OK;
+  }
+
+  nsAutoCString fontName;
+  widget->GetSystemFont(fontName);
+  aFontName.Assign(fontName);
+  return NS_OK;
+}
+

--- a/dom/interfaces/base/nsIDOMWindowUtils.idl
+++ b/dom/interfaces/base/nsIDOMWindowUtils.idl
@@ -1926,16 +1926,21 @@ interface nsIDOMWindowUtils : nsISupport
   void ensureDirtyRootFrame();
 
   /**
    * Capture the contents of the current WebRender frame and
    * save them to a folder relative to the current working directory.
    */
   void wrCapture();
 
+  /**
+   * NOTE: Currently works only on GTK+.
+   */
+  attribute ACString systemFont;
+
   // These consts are only for testing purposes.
   const long DEFAULT_MOUSE_POINTER_ID = 0;
   const long DEFAULT_PEN_POINTER_ID   = 1;
   const long DEFAULT_TOUCH_POINTER_ID = 2;
 
   // Match WidgetMouseEventBase::buttonType.
   const long MOUSE_BUTTON_LEFT_BUTTON   = 0;
   const long MOUSE_BUTTON_MIDDLE_BUTTON = 1;

--- a/dom/ipc/PBrowser.ipdl
+++ b/dom/ipc/PBrowser.ipdl
@@ -571,16 +571,19 @@ parent:
     /**
      * This function is used to notify the parent that it should display a
      * canvas permission prompt.
      *
      * @param aFirstPartyURI first party of the tab that is requesting access.
      */
     async ShowCanvasPermissionPrompt(nsCString aFirstPartyURI);
 
+    sync SetSystemFont(nsCString aFontName);
+    sync GetSystemFont() returns (nsCString retval);
+
 child:
     /**
      * Notify the remote browser that it has been Show()n on this
      * side, with the given |visibleRect|.  This message is expected
      * to trigger creation of the remote browser's "widget".
      *
      * |Show()| and |Move()| take IntSizes rather than Rects because
      * content processes always render to a virtual <0, 0> top-left

--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -3601,16 +3601,36 @@ void
 TabParent::SetIsActiveRecordReplayTab(bool aIsActive)
 {
   if (aIsActive != mIsActiveRecordReplayTab) {
     gNumActiveRecordReplayTabs += aIsActive ? 1 : -1;
     mIsActiveRecordReplayTab = aIsActive;
   }
 }
 
+mozilla::ipc::IPCResult
+TabParent::RecvSetSystemFont(const nsCString& aFontName)
+{
+  nsCOMPtr<nsIWidget> widget = GetWidget();
+  if (widget) {
+    widget->SetSystemFont(aFontName);
+  }
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult
+TabParent::RecvGetSystemFont(nsCString* aFontName)
+{
+  nsCOMPtr<nsIWidget> widget = GetWidget();
+  if (widget) {
+    widget->GetSystemFont(*aFontName);
+  }
+  return IPC_OK();
+}
+
 NS_IMETHODIMP
 FakeChannel::OnAuthAvailable(nsISupports *aContext, nsIAuthInformation *aAuthInfo)
 {
   nsAuthInformationHolder* holder =
     static_cast<nsAuthInformationHolder*>(aAuthInfo);
 
   if (!net::gNeckoChild->SendOnAuthAvailable(mCallbackId,
                                              holder->User(),

--- a/dom/ipc/TabParent.h
+++ b/dom/ipc/TabParent.h
@@ -623,16 +623,21 @@ protected:
   virtual mozilla::ipc::IPCResult RecvPaintWhileInterruptingJSNoOp(const LayersObserverEpoch& aEpoch) override;
 
   virtual mozilla::ipc::IPCResult RecvSetDimensions(const uint32_t& aFlags,
                                                     const int32_t& aX, const int32_t& aY,
                                                     const int32_t& aCx, const int32_t& aCy) override;
 
   virtual mozilla::ipc::IPCResult RecvShowCanvasPermissionPrompt(const nsCString& aFirstPartyURI) override;
 
+  mozilla::ipc::IPCResult
+  RecvSetSystemFont(const nsCString& aFontName) override;
+  mozilla::ipc::IPCResult
+  RecvGetSystemFont(nsCString* aFontName) override;
+
   ContentCacheInParent mContentCache;
 
   nsIntRect mRect;
   ScreenIntSize mDimensions;
   ScreenOrientationInternal mOrientation;
   float mDPI;
   int32_t mRounding;
   CSSToLayoutDeviceScale mDefaultScale;

--- a/ipc/ipdl/sync-messages.ini
+++ b/ipc/ipdl/sync-messages.ini
@@ -833,16 +833,20 @@ description =
 [PBrowser::DispatchWheelEvent]
 description =
 [PBrowser::DispatchMouseEvent]
 description =
 [PBrowser::DispatchKeyboardEvent]
 description =
 [PBrowser::EnsureLayersConnected]
 description =
+[PBrowser::SetSystemFont]
+description = test only
+[PBrowser::GetSystemFont]
+description = test only
 [PContent::SyncMessage]
 description =
 [PContent::CreateChildProcess]
 description =
 [PContent::BridgeToChildProcess]
 description =
 [PContent::OpenRecordReplayChannel]
 description = bug 1475898 this could be async

--- a/widget/PuppetWidget.cpp
+++ b/widget/PuppetWidget.cpp
@@ -1567,10 +1567,31 @@ PuppetWidget::WillDispatchKeyboardEvent(
                 TextEventDispatcher* aTextEventDispatcher,
                 WidgetKeyboardEvent& aKeyboardEvent,
                 uint32_t aIndexOfKeypress,
                 void* aData)
 {
   MOZ_ASSERT(aTextEventDispatcher == mTextEventDispatcher);
 }
 
+nsresult
+PuppetWidget::SetSystemFont(const nsCString& aFontName)
+{
+  if (!mTabChild) {
+    return NS_ERROR_FAILURE;
+  }
+
+  mTabChild->SendSetSystemFont(aFontName);
+  return NS_OK;
+}
+
+nsresult
+PuppetWidget::GetSystemFont(nsCString& aFontName)
+{
+  if (!mTabChild) {
+    return NS_ERROR_FAILURE;
+  }
+  mTabChild->SendGetSystemFont(&aFontName);
+  return NS_OK;
+}
+
 } // namespace widget
 } // namespace mozilla

--- a/widget/PuppetWidget.h
+++ b/widget/PuppetWidget.h
@@ -304,16 +304,19 @@ public:
                      nsIKeyEventInPluginCallback* aCallback) override;
 
   virtual void LookUpDictionary(
                  const nsAString& aText,
                  const nsTArray<mozilla::FontRange>& aFontRangeArray,
                  const bool aIsVertical,
                  const LayoutDeviceIntPoint& aPoint) override;
 
+  nsresult SetSystemFont(const nsCString& aFontName) override;
+  nsresult GetSystemFont(nsCString& aFontName) override;
+
   // TextEventDispatcherListener
   using nsBaseWidget::NotifyIME;
   NS_IMETHOD NotifyIME(TextEventDispatcher* aTextEventDispatcher,
                        const IMENotification& aNotification) override;
   NS_IMETHOD_(IMENotificationRequests) GetIMENotificationRequests() override;
   NS_IMETHOD_(void) OnRemovedFrom(
                       TextEventDispatcher* aTextEventDispatcher) override;
   NS_IMETHOD_(void) WillDispatchKeyboardEvent(

--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -7312,8 +7312,32 @@ nsWindow::SetCompositorHint(WindowCompos
                             cardinal_atom,
                             32, // format
                             GDK_PROP_MODE_REPLACE,
                             (guchar*)&value,
                             1);
     }
 }
 #endif
+
+nsresult
+nsWindow::SetSystemFont(const nsCString& aFontName)
+{
+    GtkSettings* settings = gtk_settings_get_default();
+    g_object_set(settings, "gtk-font-name", aFontName.get(), nullptr);
+    return NS_OK;
+}
+
+nsresult
+nsWindow::GetSystemFont(nsCString& aFontName)
+{
+    GtkSettings* settings = gtk_settings_get_default();
+    gchar* fontName = nullptr;
+    g_object_get(settings,
+                 "gtk-font-name", &fontName,
+                 nullptr);
+    if (fontName) {
+        aFontName.Assign(fontName);
+        g_free(fontName);
+    }
+    return NS_OK;
+}
+

--- a/widget/gtk/nsWindow.h
+++ b/widget/gtk/nsWindow.h
@@ -409,16 +409,19 @@ public:
     // From GDK
     int GdkCoordToDevicePixels(gint coord);
     LayoutDeviceIntPoint GdkPointToDevicePixels(GdkPoint point);
     LayoutDeviceIntPoint GdkEventCoordsToDevicePixels(gdouble x, gdouble y);
     LayoutDeviceIntRect GdkRectToDevicePixels(GdkRectangle rect);
 
     virtual bool WidgetTypeSupportsAcceleration() override;
 
+    nsresult SetSystemFont(const nsCString& aFontName) override;
+    nsresult GetSystemFont(nsCString& aFontName) override;
+
     typedef enum { CSD_SUPPORT_SYSTEM,    // CSD including shadows
                    CSD_SUPPORT_CLIENT,    // CSD without shadows
                    CSD_SUPPORT_NONE,      // WM does not support CSD at all
                    CSD_SUPPORT_UNKNOWN
     } CSDSupportLevel;
     /**
      * Get the support of Client Side Decoration by checking
      * the XDG_CURRENT_DESKTOP environment variable.

--- a/widget/nsIWidget.h
+++ b/widget/nsIWidget.h
@@ -1691,16 +1691,28 @@ class nsIWidget : public nsISupports
      */
     virtual void StopAsyncAutoscroll(const ScrollableLayerGuid& aGuid) = 0;
 
     // If this widget supports out-of-process compositing, it can override
     // this method to provide additional information to the compositor.
     virtual void GetCompositorWidgetInitData(mozilla::widget::CompositorWidgetInitData* aInitData)
     {}
 
+    /**
+     * Setter/Getter of the system font setting for testing.
+     */
+    virtual nsresult SetSystemFont(const nsCString& aFontName)
+    {
+      return NS_ERROR_NOT_IMPLEMENTED;
+    }
+    virtual nsresult GetSystemFont(nsCString& aFontName)
+    {
+      return NS_ERROR_NOT_IMPLEMENTED;
+    }
+
 private:
   class LongTapInfo
   {
   public:
     LongTapInfo(int32_t aPointerId, LayoutDeviceIntPoint& aPoint,
                 mozilla::TimeDuration aDuration,
                 nsIObserver* aObserver) :
       mPointerId(aPointerId),

--- a/widget/tests/chrome.ini
+++ b/widget/tests/chrome.ini
@@ -85,16 +85,20 @@ skip-if = toolkit != "cocoa"
 [test_secure_input.html]
 support-files = file_secure_input.html
 skip-if = toolkit != "cocoa"
 [test_native_key_bindings_mac.html]
 skip-if = toolkit != "cocoa" || verify
 [test_system_status_bar.xul]
 skip-if = toolkit != "cocoa"
 
+[test_system_font_changes.xul]
+support-files = system_font_changes.xul
+run-if = toolkit == 'gtk3' # Currently the test works on only gtk3
+
 # Windows
 # taskbar_previews.xul
 # window_state_windows.xul
 [test_chrome_context_menus_win.xul]
 skip-if = toolkit != "windows"
 support-files = chrome_context_menus_win.xul
 [test_plugin_input_event.html]
 skip-if = toolkit != "windows"

new file mode 100644
--- /dev/null
+++ b/widget/tests/system_font_changes.xul
@@ -0,0 +1,62 @@
+<?xml version="1.0"?>
+
+<!-- This Source Code Form is subject to the terms of the Mozilla Public
+   - License, v. 2.0. If a copy of the MPL was not distributed with this
+   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
+
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+
+<window id="system_font_changes_window"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        width="300"
+        height="300"
+        onload="start();">
+
+<span id="target" style="font:menu">Hello world</span>
+
+<script type="application/javascript"><![CDATA[
+  function is(condition, message) {
+    window.opener.wrappedJSObject.SimpleTest.is(condition, message);
+  }
+  function registerCleanupFunction(func) {
+    window.opener.wrappedJSObject.SimpleTest.registerCleanupFunction(func);
+  }
+
+  async function waitForFrame() {
+    return new Promise(resolve => {
+      requestAnimationFrame(resolve);
+    });
+  }
+
+  async function start() {
+    await waitForFrame();
+
+    const originalSystemFont = windowUtils.systemFont;
+    registerCleanupFunction(() => {
+      windowUtils.systemFont = originalSystemFont;
+    });
+
+    windowUtils.systemFont = 'Sans 11';
+    is(windowUtils.systemFont, 'Sans 11');
+
+    // Wait for two frames for the safety since the notification for system
+    // font changes is asynchronously processed.
+    await waitForFrame();
+    await waitForFrame();
+
+    const target = document.getElementById('target');
+    is(getComputedStyle(target).fontFamily, 'Sans');
+
+    windowUtils.systemFont = 'Serif 11';
+    is(windowUtils.systemFont, 'Serif 11');
+
+    await waitForFrame();
+    await waitForFrame();
+
+    is(getComputedStyle(target).fontFamily, 'Serif');
+
+    window.close();
+    window.opener.wrappedJSObject.SimpleTest.finish();
+  }
+]]></script>
+</window>

new file mode 100644
--- /dev/null
+++ b/widget/tests/test_system_font_changes.xul
@@ -0,0 +1,29 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css"
+                 type="text/css"?>
+<window title="Native menu system tests"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js" />
+
+<body  xmlns="http://www.w3.org/1999/xhtml">
+<p id="display"></p>
+<div id="content" style="display: none">
+</div>
+<pre id="test">
+</pre>
+</body>
+
+<script class="testbody" type="application/javascript">
+<![CDATA[
+
+SimpleTest.waitForExplicitFinish();
+window.open("system_font_changes.xul", "system_font_changes_window",
+            "chrome,width=600,height=600");
+
+]]>
+</script>
+
+</window>