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.
setTimeout() function is commonly used if you wish to have your function called once after the specified delay. The
setInterval() function is commonly used to set a delay for functions that are executed again and again, such as animations. The
setImmediate() function can be used instead of the
setTimeout(fn, 0) method to execute heavy operations in IE. The
requestAnimationFrame() function tells the browser that you wish to perform an animation and requests that the browser schedule a repaint of the window for the next animation frame.
- Calls a function or executes a code snippet after specified delay.
- Calls a function or executes a code snippet repeatedly, with a fixed time delay between each call to that function.
- Calls a function immediately after the browser has completed other operations, such as events and display updates.
- Clears the delay set by
- Cancels repeated action which was set up using
- Cancels the immediate actions, just like
Workerinstantiation. Here are some code snippets which simplify and abstract the management of daemons.
requestAnimationFrame()tells the browser that you wish to perform an animation and requests that the browser schedule a repaint of the window for the next animation frame. The method takes as an argument a callback to be invoked before the repaint.
performance.now()returns a timestamp, measured in milliseconds, accurate to one thousandth of a millisecond. This timestamp is equal to the number of milliseconds since the
navigationStartattribute of the
Date.now()returns the number of milliseconds elapsed since 1 January 1970 00:00:00 UTC.
- Workers can use timeouts and intervals just like the main thread can. This can be useful, for example, if you want to have your worker thread run code periodically instead of nonstop.
- Functions available to workers
- Basic animations
- Since we're using script to control canvas elements it's also very easy to make (interactive) animations. Unfortunately the canvas element was never designed to be used in this way (unlike Flash) so there are limitations.