Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.
From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).
From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.
Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the "Compatibility Milestones" document for more.
A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.
Stable
Provides access to XMLHttpRequest functionality.
Usage
Security Concerns
By default, the XMLHttpRequest object grants full access to any protocol scheme, which means that it can be used to read from (but not write to) the host system's entire filesystem. It also has unfettered access to any local area networks, VPNs, and the internet.
Threat Model
The XMLHttpRequest object can be used by an add-on to "phone home" and transmit potentially sensitive user data to third parties.
If access to the filesystem isn't prevented, it could easily be used to access sensitive user data, though this may be inconsequential if the client can't access the network.
If access to local area networks isn't prevented, malicious code could access sensitive data.
If transmission of cookies isn't prevented, malicious code could access sensitive data.
Attenuating access based on a regular expression may be ineffective if it's easy to write a regular expression that looks safe but contains a special character or two that makes it far less secure than it seems at first glance.
Possible Attenuations
Before being exposed to unprivileged code, this object needs to be attenuated in such a way that, at the very least, it can't access the user's filesystem. This can probably be done most securely by white-listing the protocols that can be used in the URL passed to the open() method, and limiting them to http:, https:, and possibly a special scheme that can be used to access the add-on's packaged, read-only resources.
Finally, we need to also consider attenuating http/https requests such that they're "sandboxed" and don't communicate potentially sensitive cookie information.
Globals
Constructors
XMLHttpRequest()
Creates an XMLHttpRequest. This is a constructor, so its use should always be preceded by the new operator. For more information about XMLHttpRequest objects, see the MDN page on Using XMLHttpRequest and the Security Concerns section in this page.
Functions
forceAllowThirdPartyCookie(xhr)
Force relevant cookies to be sent with this XMLHttpRequest even if normally they would not be.
Parameters
xhr: XMLHttpRequest
The XMLHttpRequest to allow third-party cookies for.