You should avoid using the API if at all possible. We intend to deprecate it in future releases.
If you use this API you can expect your add-on to get an extra security review by addons.mozilla.org.
This module should not be confused with the "chrome" global variable that WebExtensions can use to access APIs.
chrome module gives an Add-on SDK add-on access to the Components object, which in turn gives it access to a large set of privileged low-level Firefox APIs.
You can see an example of using this API in the XUL Migration Guide.