I had used js-ctypes on Firefox OS app long ago but I dont seem to be able to do it now. I can't even use Components, has js-ctypes been disallowed form FxOS apps?
The Components object you're referring to is only available to Firefox Add-ons or Gecko/XUL. Apps on Firefox OS are 100% HTML5, CSS3, and ECMAScript5. The only additional APIs are lower level hardware access that we're working to standardize.
If you're looking to work with C or C++ code in the browser, the preferred way is to use Emscripten to cross compile it to JS. Emscripten does have its limitation, and it depends on what you're trying to do.
If you're simply trying to work with data backed by a Typed Array, in the future we will have Typed Objects in ES7, which more usable than the current story.