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# Internals Guide #

The Internals Guide explains how to use the low-level modules provided
by the SDK.

The modules described here fall roughly into three categories:

* fundamental utilities such as
[`collection`](packages/api-utils/docs/collection.html) and
[`url`](packages/api-utils/docs/url.html). Many
add-ons are likely to want to use modules from this category.

* building blocks for higher level modules, such as [`events`](packages/api-utils/docs/events.html),
[`worker`](packages/api-utils/docs/content/worker.html), and
[`api-utils`](packages/api-utils/docs/api-utils.html).
You're more likely to use these if you are building your own modules that
implement new APIs, thus extending the SDK itself.

* privileged modules that expose powerful low-level capabilities such as
[`tab-browser`](packages/api-utils/docs/tab-browser.html),
[`xhr`](packages/api-utils/docs/xhr.html), and
[`xpcom`](packages/api-utils/docs/xpcom.html). You can
use these modules in your add-on if you need to, but should be aware that
the cost of privileged access is the need to take more elaborate security
precautions. In many cases these modules
have simpler, more restricted analogs in the high-level
[`addon-kit`](packages/addon-kit/addon-kit.html)
package (for example, [`tabs`](packages/addon-kit/docs/tabs.html) or
[`request`](packages/addon-kit/docs/request.html)).

These modules are still in active development, and we expect to make
incompatible changes to them in future releases.

### [Programming Guides](dev-guide/module-development/guides.html) ###
Documents some of the considerations involved in using the low-level modules.
In particular, it contains important information for people developing
modules which require privileged access to browser objects such as the chrome.

### [Reference](dev-guide/module-development/reference.html) ###
Detailed documentation for the low-level modules which you can use as building
blocks for your own modules. In particular, it contains modules that supply
basic services, like messaging, for higher-level modules. Many of these modules
require privileged access to the browser chrome.
