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 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.
Modify styles using stylesheet files or CSS rules.
The Style constructor creates an object that represents style modifications via stylesheet file(s) or/and CSS rules. Stylesheet file URL(s) are verified to be local to an add-on, while CSS rules are verified to be a string or array of strings.
The style created can be applied to a content by calling
attach, and removed using
detach. Those functions are part of content/mod module.
options : object
A string, or an array of strings, that represents local URI to stylesheet.
A string, or an array of strings, that contains CSS rules. Those rules are applied after the rules in the stylesheet specified with
The type of the sheet. It accepts the following values:
An array of strings that contains the CSS rule(s) specified in the constructor's option;
null if no
source option was given to the constructor. This property is read-only.
An array of strings that contains the stylesheet local URI(s) specified in the constructor's option;
null if no
uri option was given to the constructor. This property is read-only.
The type of the sheet. If no type is provided in constructor's option, it returns the default value,
"author". This property is read-only.