XUL ( ), which stands for XML User Interface Language, is a

user interface markup language A user interface markup language is a markup language that renders and describes graphical user interfaces and controls. Many of these markup languages are dialects of XML and are dependent upon a pre-existing scripting language engine, usually a ...

developed by

Mozilla Mozilla (stylized as moz://a) is a free software community founded in 1998 by members of Netscape. The Mozilla community uses, develops, spreads and supports Mozilla products, thereby promoting exclusively free software and open standards, wi ...

. XUL is an

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...

dialect for writing

graphical user interface The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows users to interact with electronic devices through graphical icons and audio indicator such as primary notation, inste ...

s, enabling developers to write user interface elements in a manner similar to web pages. XUL applications rely on the Mozilla codebase or a fork of it. The most prominent example is the Firefox web browser. However, Mozilla has reduced the usage of XUL in Firefox after their rewrite of the browser engine in version 57. This resulted in the removal of add-on customization. Firefox originally permitted add-ons to extensively alter its user interface, but this capability was removed in 2017 and replaced with the less-permissive

WebExtensions API Mozilla Firefox has features that allow it to be distinguished from other web browsers, such as Chrome and Internet Explorer. Major differences To avoid interface bloat, ship a relatively smaller core customizable to meet individual users' ne ...

. Several forks of Firefox retain support for XUL and XUL-based add-ons. Waterfox maintains a fork of the Mozilla codebase for the Waterfox Classic browser. The

Pale Moon Pale Moon is an open-source web browser with an emphasis on customization; its motto is "Your browser, Your way". There are official releases for Microsoft Windows and Linux, as well as contributed builds for various platforms. Pale Moon origi ...

browser,

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browser,

Hyperbola In mathematics, a hyperbola (; pl. hyperbolas or hyperbolae ; adj. hyperbolic ) is a type of smooth curve lying in a plane, defined by its geometric properties or by equations for which it is the solution set. A hyperbola has two pieces, cal ...

operating system, and several smaller projects collectively use and maintain the Unified XUL Platform (UXP) implementation.

History

XUL was devised at

Netscape Netscape Communications Corporation (originally Mosaic Communications Corporation) was an American independent computer services company with headquarters in Mountain View, California and then Dulles, Virginia. Its Netscape web browser was onc ...

in 1997 as part of the development effort that eventually became the

codebase. XUL is similar to web technologies implemented by the

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rendering engine. When XUL was introduced, it added features beyond the then standard HTML 4. It was designed as an expansion of HTML to be used with applications rather than documents. XUL was used by various Mozilla projects,

forks In cutlery or kitchenware, a fork (from la, furca 'pitchfork') is a utensil, now usually made of metal, whose long handle terminates in a head that branches into several narrow and often slightly curved tines with which one can spear foods ei ...

of Mozilla projects, and projects closely related to Mozilla. Some software projects like Zotero began as Mozilla Firefox

extensions Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * Ex ...

and used XULRunner to become standalone desktop applications. In the early 2000s there was some interest in using XUL by other parties, including

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, but those have switched over to HTML5. The features of HTML5 that made web applications possible, also made much of XUL redundant. Mozilla released Firefox 57, also known as Firefox Quantum, in 2017. Firefox Quantum switched Gecko to a new multi-process codebase derived from their Servo research project. XUL was designed for the original single-process codebase and would have required considerable effort to re-implement. Due to the difficulty of re-creating XUL for their new engine, Mozilla removed support for legacy add-ons, including the use of custom XUL code. Mozilla switched extensions over to Google's HTML5-based WebExtensions format. In 2017, Firefox still included 289 XUL bindings for native browser components. By the end of 2019, Mozilla had removed all XUL files from their mozilla-central codebase. When Mozilla removed support from Firefox, there were still several applications using XUL. Mozilla began gradually removing XUL support from their email client, Thunderbird. They released Thunderbird 78 without support for XUL-based extensions in 2020. The SeaMonkey internet suite chose to re-implement XUL for the Firefox Quantum codebase and the suite's development has subsequently slowed. The

developers forked Mozilla's entire Firefox 52 ESR codebase to create the Unified XUL Platform (UXP) for their Goanna rendering engine. UXP maintains the traditional XUL capabilities. As a result, Pale Moon and other UXP applications have remained single-process but extensible.

Usage

XUL can only be used with the

codebase (or a fork of it) because the

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does the XUL rendering. Application programmers need to define a XUL interface as three discrete sets of components: # Content: the XUL document(s), whose elements define the layout of the

user interface In the industrial design field of human–computer interaction, a user interface (UI) is the space where interactions between humans and machines occur. The goal of this interaction is to allow effective operation and control of the machine f ...

# Skin: the CSS and image files, which define the appearance of an application # Locale: the files containing user-visible strings for easy

software localization In computing, internationalization and localization (American) or internationalisation and localisation (British English), often abbreviated i18n and L10n, are means of adapting computer software to different languages, regional peculiarities and ...

XUL defines a wide range of elements, which roughly belong to the following types: * Top-level elements:

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, page, dialog, wizard, etc. * Widgets: label, button,

text box type=search placeholder=An example text box, which can be used to search the English Wikipedia. A text box (input box), text field or text entry box is a control element of a graphical user interface, that should enable the user to input ...

, list box, combo box,

radio button A radio button or option button is a graphical control element that allows the user to choose only one of a predefined set of mutually exclusive options. The singular property of a radio button makes it distinct from checkboxes, where the user ...

check box A checkbox (check box, tickbox, tick box) is a graphical widget that permits the user to make a binary choice, i.e. a choice between one of two possible mutually exclusive options. For example, the user may have to answer 'yes' (checked) or 'n ...

tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...

menu In a restaurant, the menu is a list of food and beverages offered to customers and the prices. A menu may be à la carte – which presents a list of options from which customers choose – or table d'hôte, in which case a pre-established seque ...

, toolbar, group box, tab box, color-picker, spacer, splitter, etc. * Box model: box, grid, stack, deck, etc. * Events and scripts: script, command, key, broadcaster, observer, etc. * Data source: template, rule, etc. * Others: overlay, iframe, browser, editor, etc. The default behavior of XUL widgets can be altered with

XBL XBL (''XML Binding Language'') is an XML-based markup language for altering the behavior of XUL widgets. It was devised at Netscape in the late 1990s as an extension of XUL. The primary use of XBL was in the Firefox web browser, but Mozilla depre ...

bindings.

Example

This example shows three buttons stacked on top of each other in a vertical box container: Boxes-ex1

History

Usage

Example

''Ghostbusters'' reference

References