Found another couple of cool Firefox extensions, both useful for developers working with XHTML and mobile devices.
Small Screen Rendering XPI
http://disruptive-innovations.com/products/index.html
This extension basically lets you see what your pages will look like when rendered on a small screen mobile device.
Aardvark
http://www.karmatics.com/aardvark/
This extension lets you view how a web page has been built, and also useful for troubleshooting page layouts etc.