I do not know if this helps, but I had the same error. Alert worked in Chrome, Explorer and Safari, but not in Firefox.
In my fal was the error that I tried to send the value directly with the javascript call. Code that did not work in Firefox:

<input name="txtEmail" type="text" class="text" id="txtEmail" style="width:200px;" />
<input name="button" type="button" class="text" id="button" value="&gt;&gt;" onClick="javascript:sendMeNews(txtEmail.value);"/>

If, however, I get the value from inside the javascript function, it worked perfectly.

function sendMeNews(){
	var email = document.getElementById("txtEmail").value;
	var checkPos = email.indexOf("@")
	if (email.length==0 || checkPos=='-1'){
	alert("A valid e-mail address must be filled")
	}
	else
	{
	url="sendNews.asp?email=" + email;
	window.location.href=url;
	}
}

Hope that helps

WebGhost

My error is similar but firebug reports the following error on any use of the javascript alert function:

code: alert("Please work...");

script error: Permission denied for <http://localhost> to call method Window.alert

However, if I load the same page at the same location on IE the alert function is called successfully?

Before you cry wolf on local permissions; executing the function on the following page, hosted on a public domain, returns nothing - with the exception that firebug reports no error.

http://www.w3schools.com/JS/tryit.asp?filename=tryjs_alert

I'm using Firefox 3.6.15 on Windows XP Pro SP3

Regards, Leigh