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What is Chrome?

In a nutshell, Chrome is the user interface of Mozilla products. The user interface is composed mostly of XUL, XBL, CSS, and JS files. (You can make changes to these files without needing to recompile the Mozilla project you're working on - which is an enormous timesaver, especially for people who are just starting out on Mozilla-based development).

If you look in the chrome subdirectory of your installation, you'll find .jar files - these contain the chrome files that make up the user interface. These jar files are actually just ZIP archives. Extensions are (usually) chrome applets. 90% of an extension, typically is chrome.

In the XUL world, Chrome is the top level window which contains groups of UI elements of various types. Example of chrome are the browser window and dialog window.

Chrome in depth

Chrome Providers

The chrome obtains its UI description from four types of providers:

  • Content provider
Provides the skeleton, i.e., the menus, command buttons
  • Skin provider
Supplies the look-and-feel of the chrome
  • Platform provider
Gives platform dependent UI description
  • Locale provider
Offers the language and culture sensitive resources

Chrome URLs

To maximize the flexibility, Mozilla introduces a new technology called "Configurable Chrome" (Refer to Configurable Chrome by Benjamin Smedberg <>). A XUL file can be written in such a fashion that its chrome description comes from a mixture of physical sources, either locally, remotely, or a combination of both. An end user may customize her/his configuration to pick up any of the source type from his favor chrome providers.

To achieve this, chrome type URLs must be used to reference external sources; such as CSS files and JavaScript files. A Chrome URL points to a file registered within the Mozilla product (it could be e-mail, calendar, browser etc.)

The general form of chrome URLs look like this [Refer to XULPlanet - The Chrome URL for more details]:

  • The "WindowType" is the window type of the chrome. Possible values are "navigator", "messenger", etc. The "ProviderType" is one of the four providers: content, skin, platform, or locale. Examples of the provider names, "ProviderName", are Mozilla, Mozillazine, xyzOrg, myISP, and etc. (Refer to XUL - Creating Localizable XML GUI for illustrations).

Below: Chrome registry - mapping chrome providers to URLs Chrome.JPG

Chrome Registry

This aspect requires sufficient understanding of Chrome.

Acoording to WierdAl (#developers), Firefox uses the toolkit approach: Chrome Registration - MDC whereas SeaMonkey uses the old way (xpfe).

Code Registry Example

1. content       necko                   jar:comm.jar!/content/necko/ xpcnativewrappers=yes
2. locale	       necko       en-US       jar:en-US.jar!/locale/en-US/necko/
3. content       xbl-marquee             jar:comm.jar!/content/xbl-marquee/
4. content       pipnss                  jar:pipnss.jar!/content/pipnss/
5. locale        pipnss      en-US       jar:en-US.jar!/locale/en-US/pipnss/
6. # Firefox-only
7. overlay chrome://browser/content/pageInfo.xul          chrome://pippki/content/PageInfoOverlay.xul application={ec8030f7-c20a-464f-9b0e-13a3a9e97384}
8. overlay chrome://communicator/content/pref/preftree.xul chrome://pippki/content/PrefOverlay.xul
9. overlay chrome://navigator/content/pageInfo.xul         chrome://pippki/content/PageInfoOverlay.xul
10. content       pippki                  jar:pippki.jar!/content/pippki/ xpcnativewrappers=yes
11. locale        pippki      en-US       jar:en-US.jar!/locale/en-US/pippki/
12. content       global-platform         jar:toolkit.jar!/content/global-platform/ platform
13. skin          global      classic/1.0 jar:classic.jar!/skin/classic/global/
14. override chrome://global/content/netError.xhtml jar:embedder.jar!/global/content/netError.xhtml
15. content       inspector               jar:inspector.jar!/content/inspector/ xpcnativewrappers=no
  • Line 1:
    content packagename uri/to/files/ [flags]
    • This code will register the package called necko when resolving the URL jar:comm.jar!/content/necko/ with the xpcnativewrappers flag on

  • Line 5:
    locale packagename localename uri/to/files/ [flags]
    • This code will register a locale package called pinpnss with the localename en-US when resolving the URL jar:en-US.jar!/locale/en-US/pipnss/
    • NOTE: If there are more than one locale file register for the package, chrome will select the best-fit locale for the URL.

  • Line 6:
    # this line is a comment - you can put whatever you want here
    • To comment your code, put '#' in the beginning of the line.

  • Line 7:
    overlay chrome://URI-to-be-overlayed chrome://overlay-URI [flags]
    • The prupose of this code is to overlaying one chrome on top of the other. The application={ec8030f7-c20a-464f-9b0e-13a3a9e97384} is a flag that specific which application will have this overlay.

  • Line 13:
    skin packagename skinname uri/to/files/ [flags]
    • This will register a skin package gobal with the skinname classic/1.0 when resolving the URL jar:classic.jar!/skin/classic/global/
    • NOTE: If more than one skin register for the package, chrome will select the best-fit skin for the URL.

  • Line 14:
    override chrome://package/type/original-uri.whatever new-resolved-URI [flags]
    • The purpose of this is to override a chrome file provided by the application or XULRunner with a new extension or embedder.

Additional Info

Starting Out

WierdAl in #developers said he started doing the development work by obtaining Gerv's Patch Maker and looking in Bugzilla for a bug with the words "good first bug" in the Status Whiteboard. WierdAl also has a blog on his development:

Chrome Tutorials

Here are tutorials on how to create user interface parts using Chrome:


WierdAl in #developers

XUL - Creating Localizable XML GUI

Configurable Chrome    by Benjamin Smedberg <>

Chrome Registration

1.1 Introduction