Final answer:
The onchange event not working with apex:actionsupport in IE9 is likely a compatibility issue. Using jQuery or implementing specific workarounds for IE9 can help ensure consistent behavior across browsers. If issues persist, upgrading or switching to a modern browser that fully supports Visualforce is advised.
Step-by-step explanation:
The onchange event not working with apex:actionsupport in IE9 can be a compatibility issue, as IE9 and earlier versions of Internet Explorer have been known to have limitations with JavaScript and various HTML5 features that are commonly used in modern web applications. Since Salesforce Visualforce pages use apex:actionsupport to handle Ajax requests, it's important to ensure that the JavaScript you're using is compatible with the browser's capabilities.
One common solution is to use a JavaScript library like jQuery, which can handle browser compatibility issues and provide consistent behavior across different browsers. Alternatively, you might need to implement a workaround for the specific behavior of IE9, such as using different event handlers, or updating the browser's settings to properly execute the Ajax requests triggered by the onchange event.
If these solutions do not work, considering upgrading the browser or urging users to use a more modern browser that fully supports Visualforce functionality could be the last resort.
.