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 (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.

A block of JavaScript code is generally executed synchronously. But there are some JavaScript native functions (timers) which allow us to delay the execution of arbitrary instructions:

The 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 setTimeout().
Cancels repeated action which was set up using setInterval().
Cancels the immediate actions, just like clearTimeout() for setTimeout().
Using JavaScript timers within animations (Javascript Daemons Management)
In Computer science a daemon is a task that runs as a background process, rather than being under the direct control of an interactive user. In JavaScript programming language, all daemons are processes created by JavaScript timers or by a Worker instantiation. 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. returns a timestamp, measured in milliseconds, accurate to one thousandth of a millisecond. This timestamp is equal to the number of milliseconds since the navigationStart attribute of the performance.timing interface. returns the number of milliseconds elapsed since 1 January 1970 00:00:00 UTC.
Using JavaScript timers within workers
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
In addition to the standard JavaScript set of functions (such as String, Array, Object, JSON etc), there are a variety of functions available from the DOM to workers. This article provides a list of those.
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.
The Timer.jsm JavaScript code module contains pure-JavaScript implementations of setTimeout and clearTimeout that are compatible with the DOM window functions, but that can be used by code that does not have access to a DOM window (for example, JavaScript code modules or content frame scripts).

See also

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 Last updated by: FranDias,