http://www.asp.net/AJAX/Documentation/Live/BrowserCompatibilityForASPNETAJAX.aspx
Supported Browsers
1) Microsoft Internet Explorer 6.0 or later versions.
2) Mozilla Firefox version 1.5 or later versions.
3) Opera version 9.0 or later versions.
4) Apple Safari version 2.0 or later versions.
Security and Privacy Settings
The following table lists required browser security and privacy settings for both user browsing and site development. In all cases, the recommended settings are the default settings for that browser.
1) Internet Explorer 6
Make sure that the Internet Zone in the Security Zones settings is set to Medium.
2) Internet Explorer 7
Make sure that the Internet Zone in the Security Zones settings is set to Medium-High.
3) FireFox 1.5 or later versions
In the Tools menu under Options, make sure that Enable JavaScript is selected.
4) Opera 9.0 or later versions
In the Tools menu under Quick preferences, make sure that Enable JavaScript is selected.
5) Safari 2.0 or later versions
Click Safari, Preferences, Security, and then Web Content and make sure that Enable JavaScript is selected.