--- a/toolkit/components/extensions/ExtensionContent.jsm
+++ b/toolkit/components/extensions/ExtensionContent.jsm
@@ -204,21 +204,16 @@ Script.prototype = {
    *        given state exactly matches the state that triggered the
    *        change.
    * @param {string} when
    *        The document's current load state, or if triggered by a
    *        document state change, the new document state that triggered
    *        the injection.
    */
   tryInject(window, sandbox, shouldRun, when) {
-    if (!this.matches(window)) {
-      this.deferred.reject({message: "No matching window"});
-      return;
-    }
-
     if (shouldRun("document_start")) {
       let winUtils = window.QueryInterface(Ci.nsIInterfaceRequestor)
                            .getInterface(Ci.nsIDOMWindowUtils);
 
       let {cssURLs} = this;
       if (cssURLs.length > 0) {
         let method = this.remove_css ? winUtils.removeSheetUsingURIString : winUtils.loadSheetUsingURIString;
         for (let url of cssURLs) {