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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 addons.mozilla.org (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.


Information about the application on which your add-on is running.

With the exception of ids, each of these properties exposes the attribute of the same name on the nsIXULAppInfo interface. For more information, see the MDN documentation.




Checks whether the host application is the given application.


name : string
A host application name.


boolean : True if the host application is name and false otherwise.


Checks whether the host application is one of the given applications.


names : array
An array of host application names.


boolean : True if the host application is one of the names and false otherwise.

versionInRange(version, lowInclusive, highExclusive)

Compares a given version to a version range. See the MDN documentation for details on version comparisons.


version : string
The version to compare.

lowInclusive : string
The lower bound of the version range to compare. The range includes this bound.

highExclusive : string
The upper bound of the version range to compare. The range does not include this bound.


boolean : True if version falls in the given range and false otherwise.



The GUID of the host application. For example, for Firefox this value is "{ec8030f7-c20a-464f-9b0e-13a3a9e97384}".


The host application name. The possible values here are:

  • "Firefox"
  • "Fennec"
  • "Mozilla"
  • "SeaMonkey"
  • "Sunbird"
  • "Thunderbird"

"Firefox"and"Fennec"` are the most commonly used values.


The host application version.


The Gecko/XULRunner platform version.


A mapping of application names to their IDs. For example, ids["Firefox"] == "{ec8030f7-c20a-464f-9b0e-13a3a9e97384}".

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 Contributors to this page: wbamberg
 Last updated by: wbamberg,