O Prism é despojado do Firefox sob o capô, por isso está disponível em 2 versões, plug-in FIREFOX ou autônomo (FF sob o capô).
Você não pode alterá-lo na versão atual.
Aqui estão mais detalhes:
Prism is a simple XULRunner based browser that hosts web applications without the normal web browser user interface. Prism is based on a concept called Site Specific Browsers (SSB). An SSB is an application with an embedded browser designed to work exclusively with a single web application. It doesn’t have the menus, toolbars and accoutrements of a normal web browser. Some people have called it a "distraction free browser" because none of the typical browser chrome is used. An SSB also has a tighter integration with the OS and desktop than a typical web application running through a web browser.