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 ...

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, w ...

. 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 ...

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, inst ...

s, enabling developers to write user interface elements in a manner similar to web pages. XUL

applications Application may refer to: Mathematics and computing * Application software, computer software designed to help the user to perform specific tasks ** Application layer, an abstraction layer that specifies protocols and interface methods used in a c ...

rely on the Mozilla

codebase In software development, a codebase (or code base) is a collection of source code used to build a particular software system, application, or software component. Typically, a codebase includes only human-written source code files; thus, a codeb ...

or a

fork 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 tine (structural), tines with which one ...

of it. The most prominent example is the

Firefox Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. It uses the Gecko rendering engine to display web pages, which implements current ...

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. Several

s of Firefox retain support for XUL and XUL-based add-ons.

Waterfox Waterfox is an open-source web browser that is forked from Firefox and developed by System1. There are official Waterfox releases for Windows, macOS, and Linux. Divisions Waterfox Waterfox shares core features and technologies like the Gecko b ...

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,

Basilisk In European bestiaries and legends, a basilisk ( or ) is a legendary reptile reputed to be a serpent king, who causes death to those who look into its eyes. According to the ''Naturalis Historia'' of Pliny the Elder, the basilisk of Cyrene i ...

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, ca ...

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

History

XUL was devised at Netscape in 1997 as part of the development effort that eventually became the

. XUL is similar to web technologies implemented by the Gecko rendering engine. When XUL was introduced, it added features beyond the then standard

HTML 4 The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaScript ...

. It was designed as an expansion of HTML to be used with applications rather than documents. XUL was used by various Mozilla projects, forks of Mozilla projects, and projects closely related to Mozilla. Some software projects like

Zotero Zotero () is a free and open-source reference management software to manage bibliographic data and related research materials, such as PDF files. Features include web browser integration, online syncing, generation of in-text citations, footno ...

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 * E ...

and used

XULRunner XULRunner is a discontinued, packaged version of the Mozilla platform to enable standalone desktop application development using XUL, developed by Mozilla. It replaced the ''Gecko Runtime Environment'', a stalled project with a similar purpose. T ...

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

Amazon Amazon most often refers to: * Amazons, a tribe of female warriors in Greek mythology * Amazon rainforest, a rainforest covering most of the Amazon basin * Amazon River, in South America * Amazon (company), an American multinational technolog ...

, but those have switched over to

HTML5 HTML5 is a markup language used for structuring and presenting content on the World Wide Web. It is the fifth and final major HTML version that is a World Wide Web Consortium (W3C) recommendation. The current specification is known as the HTML ...

. 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 Servo may refer to: Mechanisms * Servomechanism, or servo, a device used to provide control of a desired operation through the use of feedback ** AI servo, an autofocus mode ** Electrohydraulic servo valve, an electrically operated valve that c ...

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 Thunderbird, thunder bird or thunderbirds may refer to: * Thunderbird (mythology), a legendary creature in certain North American indigenous peoples' history and culture * Ford Thunderbird, a car Birds * Dromornithidae, extinct flightless birds ...

. They released Thunderbird 78 without support for XUL-based extensions in 2020. The

SeaMonkey SeaMonkey is a free and open-source Internet suite. It is the continuation of the former Mozilla Application Suite, based on the same source code, which itself grew out of Netscape Communicator and formed the base of Netscape 6 and Netscape ...

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 A goanna is any one of several species of lizards of the genus '' Varanus'' found in Australia and Southeast Asia. Around 70 species of ''Varanus'' are known, 25 of which are found in Australia. This varied group of carnivorous reptiles ranges ...

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

(or a

of it) because the Gecko

engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ...

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 an ...

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

window A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent mat ...

, page,

dialog Dialog is an online information service owned by ProQuest, who acquired it from Thomson Reuters in mid-2008. Dialog was one of the predecessors of the World Wide Web as a provider of information, though not in form. The earliest form of the Dial ...

, wizard, etc. * Widgets: label,

button A button is a fastener that joins two pieces of fabric together by slipping through a loop or by sliding through a buttonhole. In modern clothing and fashion design, buttons are commonly made of plastic but also may be made of metal, wood, o ...

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 A combo box is a commonly used graphical user interface widget (or control). Traditionally, it is a combination of a drop-down list or list box and a single-line editable textbox, allowing the user to either type a value directly or select a v ...

, radio button,

check box A checkbox (check box, tickbox, tick box) is a graphical widget that permits the user (computing), 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 ...

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 The toolbar, also called a bar or standard toolbar (originally known as ribbon) is a graphical control element on which on-screen icons can be used. A toolbar often allows for quick access to functions that are commonly used in the program. Some ...

, 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 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