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.
The 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.
chrome is a built-in pseudo module of the toolkit loader. See the chrome authority tutorial for more details.
You can see an example of using this API in the XUL Migration Guide.