Unfortunately, any mention of creating a lean UI based on HTML5 creates a huge debate, nee a holy war, in the Firefox community because this is deemed to reduce the ability to extend the Firefox UI through extensions. Please refer to [1] for a protracted argument on why WebExtensions (or similar framework) are a step-back/inferior because they don't allow for extending the Firefox UI the way XUL-based extensions do. There are a number of power-users and vocal community members who believe that Firefox must have XUL-based extensions otherwise it becomes an inferior clone of Chrome (bollocks if you ask me).

[1] http://forums.mozillazine.org/viewtopic.php?f=23&t=2956625