We are planning to deprecate the use by Firefox add-ons of the techniques described in this document.

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 instead.

Add-ons developed using these techniques might not work with multiprocess Firefox (e10s), which is already the default in Firefox Nightly and Firefox Developer Edition, and will soon be the default in Beta and Release versions of Firefox. We have documentation on making your add-ons multiprocess-compatible, but it will be more future-proof for you to migrate to WebExtensions.

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".

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