Although the browser version to version upgrade their interfaces to make them easier to use and powerful will always be ideas that you want to implement, but never arrive. It is for this reason and more, which Mozilla has started a new experiment called chromeless, which has little to do with the Google browser, and much to let the user design their own interface for Firefox.
As explained in his blog, the Firefox interface is designed using XUL and XPCOM, which allows developers to deploy new high efficiency. But of course, that does not mean that it is easy to any user of Firefox developer and want to change some aspects of the browser interface. Functional version available, uses iFrames instead of XUL elements and therefore not free to expect the final product.
From an HTML file
For now, try the current version pre-alpha, which is a mixture of Lab Cuddlefish Atul Varma, Jetpack SDK and XUL Runner. And although not exactly what they want, they can show they want to reach the goal. The application runs from an HTML file and has special privileges such as access modules CommonJS by Jetpack platform. Anyway, it is clear that this is just temporary until they can go further in development. What is lacking?
They are currently testing a pre-alpha version able to load HTML pages and render the browser interface. The next step is to add APIs (application programming interface) that correspond to specific user needs to build their browsers. Then, investigate ways to integrate security features and finally create a SDK (software development kit) available for anyone to edit the interface of Firefox can do it smoothly.