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. 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 in depth

Chrome URLs

To maximize the flexibility, Mozilla introduces a new technology called "Configurable Chrome" (Refer to Configurable Chrome by Benjamin Smedberg <benjamin@smedbergs.us>). 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]:

chrome://WindowType/ProviderType/[ProviderName/]

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).

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).

Additional Info

Code Example

content       necko                   jar:comm.jar!/content/necko/ xpcnativewrappers=yes
locale	       necko       en-US       jar:en-US.jar!/locale/en-US/necko/
content       xbl-marquee             jar:comm.jar!/content/xbl-marquee/
content       pipnss                  jar:pipnss.jar!/content/pipnss/
locale        pipnss      en-US       jar:en-US.jar!/locale/en-US/pipnss/
# Firefox-only
overlay chrome://browser/content/pageInfo.xul           chrome://pippki/content/PageInfoOverlay.xul application={ec8030f7-c20a-464f-9b0e-13a3a9e97384}
overlay chrome://communicator/content/pref/preftree.xul chrome://pippki/content/PrefOverlay.xul
overlay chrome://navigator/content/pageInfo.xul         chrome://pippki/content/PageInfoOverlay.xul application=seamonkey@applications.mozilla.org
content       pippki                  jar:pippki.jar!/content/pippki/ xpcnativewrappers=yes
locale        pippki      en-US       jar:en-US.jar!/locale/en-US/pippki/
content       global-platform         jar:toolkit.jar!/content/global-platform/ platform
skin          global      classic/1.0 jar:classic.jar!/skin/classic/global/
override chrome://global/content/netError.xhtml jar:embedder.jar!/global/content/netError.xhtml
content       inspector               jar:inspector.jar!/content/inspector/ xpcnativewrappers=no

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: