diff --git a/browser/components/preferences/advanced.js b/browser/components/preferences/advanced.js
index ebd73efe8c..84be75b0fd 100644
--- a/browser/components/preferences/advanced.js
+++ b/browser/components/preferences/advanced.js
@@ -222,7 +222,7 @@ var gAdvancedPane = {
    */
   updateHardwareAcceleration: function()
   {
-    if (AppConstants.platform = "win") {
+    if (AppConstants.platform == "win") {
       var fromPref = document.getElementById("layers.acceleration.disabled");
       var toPref = document.getElementById("gfx.direct2d.disabled");
       toPref.value = fromPref.value;
diff --git a/browser/components/preferences/in-content/advanced.js b/browser/components/preferences/in-content/advanced.js
index 448cfeb6cb..0de3b2ed50 100644
--- a/browser/components/preferences/in-content/advanced.js
+++ b/browser/components/preferences/in-content/advanced.js
@@ -209,7 +209,7 @@ var gAdvancedPane = {
    */
   updateHardwareAcceleration: function()
   {
-    if (AppConstants.platform = "win") {
+    if (AppConstants.platform == "win") {
       var fromPref = document.getElementById("layers.acceleration.disabled");
       var toPref = document.getElementById("gfx.direct2d.disabled");
       toPref.value = fromPref.value;