bug 828508 - use higher-res favicons for tab titles in hidpi mode if available. r=dao

browser/base/content/tabbrowser.xml

@@ -694,9 +694,15 @@
                                                                this.mFaviconService.FAVICON_LOAD_NON_PRIVATE);
             }
 
-            if ((browser.mIconURL || "") != aTab.getAttribute("image")) {
-              if (browser.mIconURL)
-                aTab.setAttribute("image", browser.mIconURL);
+            let sizedIconUrl = browser.mIconURL || "";
+            if (sizedIconUrl) {
+              let size = Math.round(16 * window.devicePixelRatio);
+              sizedIconUrl += (sizedIconUrl.contains("#") ? "&" : "#") +
+                              "-moz-resolution=" + size + "," + size;
+            }
+            if (sizedIconUrl != aTab.getAttribute("image")) {
+              if (sizedIconUrl)
+                aTab.setAttribute("image", sizedIconUrl);
               else
                 aTab.removeAttribute("image");
               this._tabAttrModified(aTab);