Custom Buttons² Firefox extension buttons can be written to do various tasks that the user may require. The buttons are coded in JavaScript. Many bookmarklets can easily be converted to buttons.

Extension developers will find this extension quite useful in the development cycle. JavaScript code can be prototyped in a custombutton, avoiding the reload chrome step till the code is proven. Then the code can be moved to the extension

From the IBM website:

The page-reload cycle presents one of the biggest usability obstacles in Web application development and is a serious challenge for Java™ developers. In this series, author Philip McCarthy introduces a groundbreaking approach to creating dynamic Web application experiences. Ajax (Asynchronous JavaScript and XML) is a programming technique that lets you combine Java technologies, XML, and JavaScript for Java-based Web applications that break the page-reload paradigm.