Ajax Browser support
Ajax supports all kind of browsers with small conflicts.
Older browsers like Internet Explorer 5.0 6.0 didn't support xmlHttpRequest directly so instead they are using ActiveXObject which is implemented in Explorer 5.0 and 6.0.

Latest Browsers like Firefox, Opera, Chrome, Safari and IE 8 implemented the xmlHttpRequest to support Ajax.

  At first website must satisfy a Compatible view in all kind of browsers. If you use ajax in your web Application then you have to use two kind of codes for compatibility.

Older Browsers like IE5.0 and IE6.0
xmlhttp = new ActiveXObject("Microsoft.XMLHttpRequest");

For Latest Browsers
xmlhttp = new XMLHttpRequest();

So using the above code creates an object for all kind of browsers. No more compatible issue in using Ajax.

Code for Checking Browser Compatibility
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
{// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");