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.
Experimental
Helper functions for working with stylesheets.
Globals
Functions
loadSheet(window, uri, type)
Synchronously loads a style sheet from uri and adds it to the list of additional style sheets of the document. The sheets added takes effect immediately, and only on the document of the window given.
Parameters
window : nsIDOMWindow
uri : string,
nsIURI}
type : string
The type of the sheet. It accepts the following values: "agent", "user" and "author". If not provided, the default value is "author".
removeSheet(window, uri, type)
Remove the document style sheet at sheetURI from the list of additional style sheets of the document. The removal takes effect immediately.
Parameters
window : nsIDOMWindow
uri : string,
nsIURI}
type : string
The type of the sheet. It accepts the following values: "agent", "user" and "author". If not provided, the default value is "author".
isTypeValid(type)
Verifies that the type given is a valid stylesheet's type. The values considered valid are: "agent", "user" and "author".
Parameters
type : string
The type of the sheet.
Returns
boolean : true if the type given is valid, otherwise false.