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.
Unstable
Access to information about Firefox's runtime environment. All properties exposed are read-only.
For more information, see nsIXULRuntime.
Globals
Properties
inSafeMode
This value is true if Firefox was started in safe mode, otherwise false.
OS
A string identifying the current operating system. For example, . "WINNT", "Darwin", or "Linux". See OS_TARGET for a more complete list of possible values.
processType
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. |
widgetToolkit
A string identifying the target widget toolkit in use.
XPCOMABI
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".