XHTML Mobile Profile allows Mozilla Firefox to render HTML content with the mime-type of application/vnd.wap.xhtml+xml. For any developer/tester working with mobile applications, this provides an alternative to using a device emulator. (Although nothing beats testing with actual devices.) If the application uses device detection to provide correctly rendered content to a device, use the Modify Headers extension to test the markup targeted at a specific device.
Firefox does not natively support the mime-type application/vnd.wap.xhtml+xml. This is one of the possible mime-types for XHTML Mobile Profile. This extension adds support for this mime-type (The other, application/xhtml+xml, is supported natively by Firefox).