Filename extension .xul
Internet media type application/vnd.mozilla.xul+xml
Developed by Mozilla Foundation
Type Markup language
Opens with Mozilla Firefox
Mozilla Thunderbird
File formats category - v  e   edit
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XUL (pronounced zool ([zu:l]), XML User Interface Language), is an XML user interface markup language developed by the Mozilla project which operates in Mozilla cross-platform applications such as Mozilla Firefox and Flock.


XUL relies on multiple existing web standards and technologies, including CSS, JavaScript, and DOM. Such reliance makes XUL relatively easy to learn for people with a background in web-programming and design.

XUL has no formal specification and does not inter-operate with non-Gecko implementations. However, it uses an open source implementation of Gecko, tri-licensed under the GPL, LGPL, and MPL.[1]

Mozilla provides experimental XULRunner builds to let developers build their applications on top of the Mozilla application framework and XUL in particular.

XUL documentsEdit

Programmers typically define a XUL interface as three discrete sets of components:

  1. content: the XUL document(s), whose elements define the layout of the user interface
  2. skin: the CSS and image files, which define the appearance of an application
  3. locale: the files containing user-visible strings for easy software localization

XUL elementsEdit

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

Top-level elements
e.g., window, page, dialog, wizard, etc.
e.g., label, button, text box, list box, combo box, radio button, check box, tree, menu, toolbar, group box, tab box, color picker, spacer, splitter, etc.
Box model
e.g., box, grid, stack, deck, etc.
Events and scripts
e.g., script, command, key, broadcaster, observer, etc.
Data source
e.g., template, rule, etc.
e.g., overlay (analogous to SSI, but client-side and more powerful, with higher performance), iframe, browser, editor, etc.

One can use elements from other applications of XML within XUL documents, such as XHTML, SVG, and MathML.

Mozilla added some common widgets — <scale/> (sometimes called "slider"), <textbox type="number"/> (spinbox), time and date pickers — during the Gecko 1.9 development-cycle.[2]

XUL applicationsEdit

While XUL serves primarily for creating the Mozilla applications and their extensions, it may also feature in web applications transferred over HTTP. The Mozilla Amazon Browser, a former XUL application of this type and well-known in its day[3], provided a rich interface for searching books at

However, many of the powerful features of Mozilla such as privileged XPCOM objects remain unavailable to unprivileged XUL documents unless the script has a digital signature, and unless the user obtains grants of certain privileges to the application. Such documents also suffer from various limitations of the browser, including the inability to load remote XUL, DTD, and RDF documents.

Mozilla-programmers sometimes refer to XUL applications running locally as "chrome".[4]


This example shows 3 buttons stacked on top of each other in a vertical box container:[5]

<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
<window id="vbox example" title="Example"
  <button id="yes" label="Yes"/>
  <button id="no" label="No"/>
  <button id="maybe" label="Maybe"/>

See alsoEdit


External linksEdit

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