Extensions allow programmers to add new features to Mozilla applications or allow existing features to be modified. Typically, extensions modify the “chrome” of their target application—the user interface and the scripts that add functionalty to that interface. However, it is also possible for extensions to include compiled code in the form of XPCOM components.
You will almost certainly need to use the following technologies, although it is not necessary to be an expert in them. You may be able to pick up much of what you need just by examining the source code for other extensions.
- XUL (XML User-Interface Language). Used to define the onscreen layout of the UI and to attach scripts and style to the interface.
- DOM (Document Object Model). Used to manipulate XUL in real-time as well as any HTML documents loaded.
- CSS (Cascading Style Sheets). Used to style the interface and to attach XBL bindings to XUL documents.
More advanced extensions may require the following technologies.
- XBL (XML Binding Language). Used to extend XUL, allowing the creation of new types of widgets and associated behaviors.
- RDF (Resource Description Framework). Used as a data-storage format in some parts of Mozilla and in some extensions.
Setting up your environment
- Please see Setting up extension development environment for development preferences you can set and extensions you can install. It also contains information about running a separate instance of Firefox using a development profile.
- Getting started with extension development contains tips on easily setting up extensions development environment in Firefox/Thunderbird 1.5 (so that you no longer have to recreate the JAR and/or XPI files each time you make a change in your extension). It is also a quickstart guide containing useful links for beginners and the obligatory helloworld extension, which can be used as a basis of your extensions.
There is a variety of third-party tutorials available which will help with general extension development or with learning specific technologies.
On this Knowledge Base and Mozilla's website:
- Getting started with extension development
- "Building an Extension" (Devmo).
- "Creating a Mozilla Extension": Walks through the process of creating a simple extension. It has a particularly useful section on XUL Overlays, which enable modifications to existing chrome without editing the actual chrome file. However, it has not yet been updated to cover the new packaging system.
On other websites:
- XULPlanet (mirror): Premier resource on the internet for XUL and XPCOM. It contains an extensive tutorial that walks you through XUL.
- HowTo (Roachfiend.com): Walks through the entire extension development process. The tutorial covers creating extensions for Firefox 1.0 and later versions. (Some content is Windows specific.)
- Nathan Yergler's minimal example extension.
- Brian Duff's Hello World tutorial.
- Jonah Bishop's Firefox Toolbar Tutorial: Creating toolbar extensions (like WebDeveloper or Googlebar) for Firefox.
- "Signing an XPI" by Pete Collins.
- "Developing Firefox Extensions with GNU/Linux" (Ars Technica's Linux.Ars column).
- A Mozilla Suite and Firefox extension development weblog.
- The Deep End: A Thunderbird weblog that focuses on extensions.
As well as web resources, there are a couple of books available describing Mozilla technologies and their usage. Both books cover a comprehensive range of Mozilla technologies. They are available both online and in a more user-friendly dead-tree format.
- Creating Applications With Mozilla. O'Reilly's reference, released at about same time as Mozilla 1.0 and avaliable online as a collection of HTML files.
- Rapid Application Development With Mozilla. Available as a set of compressed PDF files, and more up-to-date and more detailed than the O'Reilly offering. Also available as a collection of XHTML pages.
Making it better
- Be sure to make your extensions localizable: Use DTDs for XUL and properties for scripts.
- Themes apply CSS using various selectors, particularly based on
classattributes on your elements. Include these to make theme development easier.
- Try to follow the set of Extension guidelines. This can help to make all extensions act predictably, which increases usability.
- Example Code. Common development tasks and techniques.
- Example Extensions. Extensions that provide a clear demonstration of a particular technique or function.
- Cross-Version Compatibility Techniques for Extensions. (Currently applies to Mozilla 1.7.x / Moz 1.8.x only.)
- General advice for extension development, including how to avoid rebuilding JAR files and how to correctly register overlays.
- Chrome URLs. Discover the location within the installation folder of the chrome you wish to overlay.
- Mozilla development resources
- A few mozilla.org documents about extensions/themes
- Extensions section of Devmo
- MozillaZine provides a Development forum which is the place for technical questions and an Extensions forum which occasionally has technical discussion but is more useful for releasing extensions to get testing and feedback.
- The mozilla.dev.* newsgroups are the official location for code-related queries. They are particularly useful if you're implementing an XPCOM component or have a question that MozillaZine posters can't answer. See the developer forums page for a description of each newsgroup.
- IRC (see also this). Live Developers! Get satisfaction online from red hot developers! (Note: this is not a dating service.), #extdev is the place to be!
- Take a look at Dev : Project ideas to see the list of requested extensions.
- Help maintaining the Extension development knowledge base. Post your tips and code snippets for others to use. (You may use the MozillaZine forums (preferably Development or Extensions) to discuss the tips.) Also consider helping the Devmo project.
- Contribute to existing extensions.
- List ideas at (or borrow ideas from) AllYourIdeas extension category