Wierd problem with Ajax & Firefox

OK

I am returning to Ajax after 18 months doing other things.

Using VS2008, I have a very simple aspx page plus an asmx.  It works as expected in IE7.  However, in Firefox (3.0.7) I get The server method 'HelloWorld' failed., with 0 as the statusCode and no stack trace.  Here's the wierd part: if I debug the service call with Firebug, hitting F10 at a breakpoint (in runTest()) in the javascript, it returns the expected result: if I debug & hit F5 at the breakpoint I get the error.

Any clues gratefully received!

Ian

here's my code:

<!-- default.aspx -->

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="testAjax._Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtm ...