The problem of Firefox OS is the fix focus camera of over 1 meter and the max 5 mega pixel resolution. You can't get sharp barcode images. They are either blurry while to near, or too small while in focus.
I've build a Firefox OS app [1] that is theoretically enabled to scan barcodes, but it won't work with most devices out there.
It's too bad that most of the problems when scanning barcodes are related to the missing auto-focus in browsers. I really hope this gets sorted out in the near feature.
I just tried your app and reading QR codes works really well. You are using an emscripten version of ZBar, right?
I've build a Firefox OS app [1] that is theoretically enabled to scan barcodes, but it won't work with most devices out there.
[1] https://marketplace.firefox.com/app/qr-reader