Show an alert from a Firefox extension when a browser window opens

Question

I'm trying to make a very simple Firefox extension. I need it to show an alert box when the Firefox window opens. The message doesn't show up when I open the window but it does when I reload all chrome (through the Extensions Developer Add-On). My overlay file: ```

Accepted Answer

You cannot display `alert()`s before the browser window is actually loaded and displayed, because the alert dialog has to have a fully-initialized and visible parent window. Your overlay script will however be run during the load/initialization already...

The Browser Console should show an error saying `NS_ERROR_NOT_AVAILABLE: Cannot call openModalWindow on a hidden window` (but turns out, only when `alert` is called from within the `load` event handler).

So, first wait for the `load` event and then give the event loop a chance to actually show the window, e.g.

```
addEventListener("load", function() {
    setTimeout(function() { alert("something"); }, 0);
});
```

Show an alert from a Firefox extension when a browser window opens

Solution

Problem