--- a/toolkit/components/extensions/parent/ext-tabs-base.js
+++ b/toolkit/components/extensions/parent/ext-tabs-base.js
@@ -1409,20 +1409,22 @@ class WindowTrackerBase extends EventEmi
    * @throws {ExtensionError}
    *        If no window exists with the given ID and `strict` is true.
    */
   getWindow(id, context, strict = true) {
     if (id === WINDOW_ID_CURRENT) {
       return this.getCurrentWindow(context);
     }
 
-    for (let window of this.browserWindows(true)) {
-      if (this.getId(window) === id) {
-        return window;
-      }
+    let window = Services.wm.getOuterWindowWithId(id);
+    if (window && !window.closed && (window.document.readyState !== "complete"
+        || this.isBrowserWindow(window))) {
+      // Tolerate incomplete windows because isBrowserWindow is only reliable
+      // once the window is fully loaded.
+      return window;
     }
 
     if (strict) {
       throw new ExtensionError(`Invalid window ID: ${id}`);
     }
   }
 
   /**