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 [http://developer.mozilla.org/en/docs/Extensions 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 <b>browser window </b> and <b>dialog window</b>. The chrome obtains its UI description from four types of providers:

To maximize the flexibility, Mozilla introduces a new technology called "Configurable Chrome" (Refer to [http://www.mozilla.org/xpfe/ConfigChromeSpec.html 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#Chrome_Providers|chrome providers]].

This aspect of Chrome requires sufficient understanding of Chrome. Acoording to WierdAl (#developers), Firefox uses the toolkit approach: [http://developer.mozilla.org/en/docs/chrome.manifest Chrome Registration - MDC] whereas SeaMonkey uses the old way (xpfe). ==Code Registry Example==<pre>1. content necko jar:comm.jar!/content/necko/ xpcnativewrappers=yes2. 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-only7. 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.xul9. overlay chrome://navigator/content/pageInfo.xul chrome://pippki/content/PageInfoOverlay.xul application=seamonkey@applications.mozilla.org10. content pippki jar:pippki.jar!/content/pippki/ xpcnativewrappers=yes11. locale pippki en-US jar:en-US.jar!/locale/en-US/pippki/12. content global-platform jar:toolkit.jar!/content/global-platform/ platform13. skin global classic/1.0 jar:classic.jar!/skin/classic/global/14. override chrome://global/content/netError.xhtml jar:embedder.jar!/global/content/netError.xhtml15. content inspector jar:inspector.jar!/content/inspector/ xpcnativewrappers=no</pre> *Line 1: <pre>content packagename uri/to/files/ [flags]</pre>**This code will register the package called necko when resolving the URL jar:comm.jar!/content/necko/ with the xpcnativewrappers flag on  

Firefox uses *Line 5: <pre>locale packagename localename uri/to/files/ [flags]</pre>**This code will register a locale package called pinpnss with the toolkit approachlocalename 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: <pre># this line is a comment - you can put whatever you want here</pre>**To comment your code, put '#' in the beginning of the line.  *Line 7: <pre>overlay chrome://URI-to-be-overlayed chrome://overlay-URI [httpflags]</pre>**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:<pre>skin packagename skinname uri/to/files/ [flags]</pre>**This will register a skin package gobal with the skinname classic/developer1.mozilla0 when resolving the URL jar:classic.orgjar!/skin/enclassic/docsglobal/**NOTE: If more than one skin register for the package, chromewill select the best-fit skin for the URL.manifest Chrome Registration   *Line 14: <pre>override chrome://package/type/original-uri.whatever new-resolved- MDCURI [flags]</pre>**The purpose of this is to override a chrome file provided by the application or XULRunner with a new extension or embedder.

Here are the following examples of tutorials on how to create user interface parts you can create using Chrome:

http://developer.mozilla.org/en/docs/Working_with_windows_in_chrome_code http://developer.mozilla.org/en/docs/Chrome_Registration [http://www.xulplanet.com/tutorials/xultu/chromeurlintro.html http://developer1.mozilla.org/en/docs/XUL_Tutorial:Manifest_Files1 Introduction]