I don't think this is possible with the build in prompt but you can easily do so with a custom prompt window.
1) Create a XUL dialog alert_prompt.xul as follows:
<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
<dialog id="alertprompt" title="Alert"
xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
buttons="accept,cancel"
buttonlabelcancel="Cancel"
buttonlabelaccept="Save"
height="140"
width="250"
ondialogaccept="return alert_prompt.doOK();"
ondialogcancel="return alert_prompt.doCancel();">
<script type="application/javascript" src="chrome://hello/content/alert_prompt.js"/>
<dialogheader title="Timer Alert Prompt"/>
<label id="result" value="This prompt will close in 10 seconds." align="center"/>
</dialog>
2) Create a Javascript file for this XUL window alert_prompt.js
var alert_prompt = {
init : function()
{
alert_prompt.timedCount(0);
},
timedCount : function(c)
{
//update the prompt message
document.getElementById('result').value="This prompt will close in "+ (10 - c) + " seconds.";
//if 10 seconds are over close the window
if(c == 10)
{
window.close();
}
//update the counter
c=c+1;
//use the timer
t=setTimeout(
function()
{
alert_prompt.timedCount(c);
},1000)
},
doOK : function()
{
//code that you want to run when save button is pressed
return true;
},
doCancel : function()
{
//code that you want to run when cancel button is pressed
return true;
},
};
window.addEventListener("load", alert_prompt.init, false);
3) Instead of showing the alert prompt as earlier use this statement:
openDialog("chrome://hello/content/alert_prompt.xul","alert_prompt","modal");
If you want to return a value from the alert box such as which button was pressed you can do so in the same way as discussed HERE
I am not sure about the positioning of a modal window so you may want to ask that in a separate question.
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