diff --git a/addons/web_editor/static/src/js/transcoder.js b/addons/web_editor/static/src/js/transcoder.js
index 6e0097ee8fb..8dbc0ff0104 100644
--- a/addons/web_editor/static/src/js/transcoder.js
+++ b/addons/web_editor/static/src/js/transcoder.js
@@ -12,7 +12,19 @@ var getMatchedCSSRules = function (a) {
     if (!rulesCache.length) {
         var sheets = document.styleSheets;
         for(var i = sheets.length-1; i >= 0 ; i--) {
-            var rules = sheets[i].rules || sheets[i].cssRules;
+            var rules;
+            if (sheets[i].rules) {
+                rules = sheets[i].rules;
+            } else {
+                //try...catch because Firefox not able to enumerate document.styleSheets[].cssRules[] for cross-domain stylesheets.
+                try {
+                    rules = sheets[i].cssRules;
+                } catch(e) {
+                    console.warn("Can't read the css rules of: " + sheets[i].href, e);
+                    continue;
+                }
+                rules = sheets[i].cssRules;
+            }
             if (rules) {
                 for(var r = rules.length-1; r >= 0; r--) {
                     var selectorText = rules[r].selectorText;
diff --git a/addons/web_editor/static/src/js/widgets.js b/addons/web_editor/static/src/js/widgets.js
index 5926429fe73..87d985f19f4 100644
--- a/addons/web_editor/static/src/js/widgets.js
+++ b/addons/web_editor/static/src/js/widgets.js
@@ -496,7 +496,18 @@ var getCssSelectors = function(filter) {
     }
     var sheets = document.styleSheets;
     for(var i = 0; i < sheets.length; i++) {
-        var rules = sheets[i].rules || sheets[i].cssRules;
+        var rules;
+        if (sheets[i].rules) {
+            rules = sheets[i].rules;
+        } else {
+            //try...catch because Firefox not able to enumerate document.styleSheets[].cssRules[] for cross-domain stylesheets.
+            try {
+                rules = sheets[i].cssRules;
+            } catch(e) {
+                console.warn("Can't read the css rules of: " + sheets[i].href, e);
+                continue;
+            }
+        }
         if (rules) {
             for(var r = 0; r < rules.length; r++) {
                 var selectorText = rules[r].selectorText;