http://www.yourhtmlsource.com/javascript/ajax.html

Cross-browser Ajax

Unfortunately Ajax is supported slightly differently in IE than it is Safari, Opera and Mozilla-based browsers like Firefox. This leaves us with two possible routes: using code branching to send the right code to each browser based on which model they support, or using a JavaScript library that wraps up the Ajax code into a single object, and means you don’t have to worry about browser incompatibilities.
We’re going for the latter option, and will be using a JavaScript library. There are dozens of them in existence, each with their own boons and vices. Popular examples include prototype, Dojo, and the Yahoo UI library. For the duration of this tutorial, we’re going to be using a very useful library called » Sarissa. Sarissa contains methods that will create the request for us, and also methods that help with processing the XML that we receive back in the response. This means we don’t have to mess with the intricacies of Ajax, and allows our code to be quite elegant.