The trick, was not to rely on the DOMContentLoaded event to fire to do the override.  Instead, I used the DOMWillOpenModalDialog to detect the first alert().  After detection, I overrode with an alertCheck which asked the user whether or not to “suppress more dialogs”.  If the user answers in the affirmative, alert() is simply overwritten with an empty function.

Piece of cake.

A couple of issues though…

Security

In order to override the alert() function, I have to write to document.getElementById(‘content’).contentWindow.wrappedJSObject.alert.

Remember how I mentioned the distance between the Extension JavaScript, and the inline content JavaScript?  I said it felt like a security layer.

I was totally right.

Check this out. I’ll quote:

You should be aware of XPCNativeWrappers when working with untrusted content. With XPCNativeWrappers turned on (which is the default in Firefox 1.5+), your extension can safely access the DOM of the content document, but not the content JavaScript. Bypassing XPCNativeWrapper to work with content JavaScript directly can lead to security problems.

Hrmph.  So I seem to be violating some security rules here.  So maybe my approach isn’t the greatest idea.  “Mook” from irc.mozilla.net #extdev suggested looking into commonDialog.xul…but I can’t seem to wrap my head around that just yet.

Imperfections

Not sure why yet, but while I can suppress dialog floods like this:

for (i = 0; i < 10; ++i) {
  alert(i);
}

It seems to fail on this:

for (i = 0; i < 10; ++i) {
  alert(i);
  confirm(i);
}

For some reason, regardless of whether or not I choose to suppress the dialogs, they just keep coming.  It works fine when I swap out the confirm() for a second alert().  Not exactly sure why.  Yet.

Ok, so I’m going to clean the code up, and post it soon.  I’ll also post a link to a real, brutally annoying website where you can test the alertCheck extension.  Just give me a bit.