This post is from Doug Turner who has previous written about Geolocation. Doug works on Mozilla’s mobile project.

XMLHttpRequests (XHR) can be either synchronous or asynchronous. Although most people use asynchronous requests there are instances where you might want to use a synchronous request. That is, wait until the XMLHttpRequest call completes to continue executing JavaScript. In Firefox 3 and earlier the browser would still fire timer events and respond to input events during a synchronous XHR request. In Firefox 3.5 and later input events such as mouse moves and timeouts will be suspended until the synchronous request completes. This allows the synchronous request to block.

For example:

function hello() {
     alert(“hello”);
}

setTimeout(hello, 20);

var req = new XMLHttpRequest();
req.open('GET', 'http://www.mozilla.org/', false);
req.send(null);

Prior to Firefox 3.5, it was impossible to determine if the “hello” function would be invoked during or after the XHR request. This led to all sorts of timing issues in web applications that used synchronous XHR requests.

The solution to this problem has been to delay input events and timeouts until after “req.send” returns.

For more information see the two bugs on the issue.