This is an archived page. It's not actively maintained.



Support for extensions using XUL/XPCOM or the Add-on SDK was removed in Firefox 57, released November 2017. As there is no supported version of Firefox enabling these technologies, this page will be removed by December 2020.

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.

Starting from Firefox 53, no new legacy add-ons will be accepted on (AMO) for desktop Firefox and Firefox for Android.

Starting from Firefox 57, only extensions developed using WebExtensions APIs will be supported on Desktop Firefox and Firefox for Android.

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 use WebExtensions APIs if they can. See the "Compatibility Milestones" document for more information.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.



Access to information about Firefox's runtime environment. All properties exposed are read-only.

For more information, see nsIXULRuntime.




This value is true if Firefox was started in safe mode, otherwise false.


A string identifying the current operating system. For example, . "WINNT", "Darwin", or "Linux". See OS_TARGET for a more complete list of possible values.


The type of the caller's process, which will be one of these constants:

Constant Value Description
PROCESS_TYPE_DEFAULT 0 The default (chrome) process.
PROCESS_TYPE_PLUGIN 1 A plugin subprocess.
PROCESS_TYPE_CONTENT 2 A content subprocess.
PROCESS_TYPE_IPDLUNITTEST 3 An IPDL unit testing subprocess.


A string identifying the target widget toolkit in use.


A string identifying the ABI of the current processor and compiler vtable. This string takes the form <processor>-<compilerABI>, for example: "x86-msvc" or "ppc-gcc3".