Certain vulnerabilities within Ajax applications can allow malicious hackers to reek havoc with your applications. Identity theft, unprotected access to sensitive information, browser crashes, defacement of Web applications, and Denial of Service attacks are just a few of the potential disasters Ajax applications can be prone to and which developers need to guard against when building Ajax capabilities into their applications.
This DeveloperWorks article has some interesting tools. One, aSSL, is an AJAX add-on that allows you to tunnel your AJAX requests back to the server over AES encryption. Very nice. It's licensed under the MIT license.
There are also links to several Firefox add-ons and extensions, such as Session Manager, that allows you to re-establish sessions on demand.