--- a/browser/tools/mozscreenshots/mozscreenshots/extension/TestRunner.jsm
+++ b/browser/tools/mozscreenshots/mozscreenshots/extension/TestRunner.jsm
@@ -200,19 +200,20 @@ this.TestRunner = {
     if (!selectors.length) {
       throw "No selectors specified.";
     }
 
     // Set window type, default "navigator:browser"
     windowType = windowType || "navigator:browser";
     let browserWindow = Services.wm.getMostRecentWindow(windowType);
     // Scale for high-density displays
-    const scale = browserWindow.QueryInterface(Ci.nsIInterfaceRequestor)
-                        .getInterface(Ci.nsIDocShell).QueryInterface(Ci.nsIBaseWindow)
-                        .devicePixelsPerDesktopPixel;
+    const scale = Cc["@mozilla.org/gfx/screenmanager;1"]
+                    .getService(Ci.nsIScreenManager)
+                    .screenForRect(browserWindow.screenX, browserWindow.screenY, 1, 1)
+                    .defaultCSSScaleFactor;
 
     const windowLeft = browserWindow.screenX * scale;
     const windowTop = browserWindow.screenY * scale;
     const windowWidth = browserWindow.outerWidth * scale;
     const windowHeight = browserWindow.outerHeight * scale;
 
     let bounds;
     const rects = [];