--- a/toolkit/components/search/nsSearchSuggestions.js
+++ b/toolkit/components/search/nsSearchSuggestions.js
@@ -24,19 +24,20 @@ function SuggestAutoComplete() {
 SuggestAutoComplete.prototype = {
 
   _init: function() {
     this._suggestionController = new SearchSuggestionController(obj => this.onResultsReturned(obj));
     this._suggestionController.maxLocalResults = this._historyLimit;
   },
 
   get _suggestionLabel() {
-    delete this._suggestionLabel;
     let bundle = Services.strings.createBundle("chrome://global/locale/search/search.properties");
-    return this._suggestionLabel = bundle.GetStringFromName("suggestion_label");
+    let label = bundle.GetStringFromName("suggestion_label");
+    Object.defineProperty(SuggestAutoComplete.prototype, "_suggestionLabel", {value: label});
+    return label;
   },
 
   /**
    * The object implementing nsIAutoCompleteObserver that we notify when
    * we have found results
    * @private
    */
   _listener: null,