--- a/browser/components/preferences/cookies.js
+++ b/browser/components/preferences/cookies.js
@@ -728,16 +728,17 @@ var gCookiesWindow = {
   },
 
   onCookieKeyPress(aEvent) {
     if (aEvent.keyCode == KeyEvent.DOM_VK_DELETE) {
       this.deleteCookie();
     } else if (AppConstants.platform == "macosx" &&
                aEvent.keyCode == KeyEvent.DOM_VK_BACK_SPACE) {
       this.deleteCookie();
+      aEvent.preventDefault();
     }
   },
 
   _lastSortProperty: "",
   _lastSortAscending: false,
   sort(aProperty) {
     var ascending = (aProperty == this._lastSortProperty) ? !this._lastSortAscending : true;
     // Sort the Non-Filtered Host Collections