Quick reply...
Apps on Firefox OS are web apps, which can be opened on any operating system as long as it has the latest Gecko browser. However, they don't work ideally yet, not even in the latest Nightly.
Browser cannot load a webpage because, theoretically, cross-origin framing can be malicious.
Error: Load denied by X-Frame-Options: http://www.google.com/m does not permit cross-origin framing.
Camera is black, and gives no errors.
Calculator works fine, even in a non-Gecko browser.
Now, how do I download and install apps on my 'Firefox OS' phone (or computer)?
For calculator, you download html, css and js. Here, your own offline HTML calculator.
The same for camera: html, css and js.
Oh, and camera interacts with gallery, you may wish to download its three files, too.
Oh, and don't forget several images - icons for camera buttons, for instance. And be careful with where you save them - these files use relative paths to connect to each other.
And, to be comfortable, you will need to make desktop icons to open these webpages, stored on your device, in your browser.
Quite easy. Quite simple. Quite different from debian - no package manager, for now.
Best wishes.
[EDIT]Source code of all the currently available (Mozilla-created) apps, including their manifests.[/EDIT]