I think is easy to use an mvc pattern with firefox os apps, see this thread maybe you will find useful the folder structures suggested there.
I would also recommend to use a framework powered by apache cordova called ionic, ionic also organize your app in a similar way as MVC pattern.
The lack of server side scripting in firefox os is not a limitation to organize your project in a MVC way, just keep in mind the pattern basis, divide your views and logic.
here an example using angular.js as your client side MVC framework that could help to organize your app in a similar way as MVC pattern:
your-app-folder
├── index.html
├── manifest.webapp
├── js
│ ├── controllers
│ │ └── main.js
│ │ └── ...
│ ├── directives
│ │ └── yourDirective.js
│ │ └── ...
│ ├── filters
│ │ └── yourFilter.js
│ │ └── ...
│ ├── services
│ │ └── yourService.js
│ │ └── ...
│ ├── libs
│ │ ├── jquery.min.js
│ │ └── angular.min.js
│ └── app.js
├── css
│ └── ...
└── views
├── login.html
└── ...