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:

<?xml version="1.0"?>
<overlay xmlns=".is.only.xul">
    <script type="application/x-javascript" src="chrome://adTest/content/alert.js" />
</overlay>

My script file:

alert("HI!");

My chrome.manifest file:

content adTest content/ contentaccessible=yes
overlay chrome://browser/content/browser.xul chrome://adTest/content/adTestOverlay.xul

I'm pretty sure the rest of the code is correct because I've added XUL elements for testing purposes and everything worked apart from the alert box.

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:

<?xml version="1.0"?>
<overlay xmlns="http://www.mozilla/keymaster/gatekeeper/there.is.only.xul">
    <script type="application/x-javascript" src="chrome://adTest/content/alert.js" />
</overlay>

My script file:

alert("HI!");

My chrome.manifest file:

content adTest content/ contentaccessible=yes
overlay chrome://browser/content/browser.xul chrome://adTest/content/adTestOverlay.xul

I'm pretty sure the rest of the code is correct because I've added XUL elements for testing purposes and everything worked apart from the alert box.

• javascript
• firefox
• firefox-addon
• xul

• Can you put content of "chrome.manifest" here? – Hassan Khodadadeh Commented Jul 10, 2014 at 15:43
• I have added chrome.manifest. – user3399738 Commented Jul 10, 2014 at 15:50

2 Answers 2

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);
});

Can also do:

Services.prompt.alert(null, 'title of alert', 'alert msg');

In place of null you can supply window and that will make that window modal and unselectable while that alert is showing (just like normal alert)