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Hack 84. Do Groundwork for Extension Development

Understand how extensions, themes, and locales fit into the Firefox architecture.

Chapter 7 introduced the key ideas of chrome, chrome: URLs, JAR files, and packages [Hack #75] . Those concepts also apply to more formal chrome enhancement projects. Such projects also require the concepts of extensions, themes, locales, XPIs, and XPInstall. This hack describes those concepts in more detail. It's all about collecting together the jigsaw pieces in preparation for solving the puzzle.

There are some basic differences between quick hacks to the chrome and fully prepared extensions. These are the main advantages of extensions:

  • They have a formal concept of version.

  • They can be updated after they are installed.

  • They can be updated across the Web from a web site.

  • If Firefox is upgraded, installed extensions are not lost.

  • Registration information exposes details of extensions to scripts.

  • Extensions are, by default, stored per-profile in the profile area.

  • Firefox's -safe-mode option allows extensions to be disabled easily.

8.2.1. Spot Extension Files

All installed extensions live in the extensions directory, either in the profile area or in the install area. In that directory are two important files. Extensions.rdf is a small database of all the currently installed extensions. The other file, usually named something like installed-extensions.txt, lists the extensions that were discovered the last time Firefox did a big audit of all the extensions in the extensions directory. This happens when the installed-extensions.txt file is updated, through the same approach used for the install area's installed-chrome.txt file [Hack #75] .

Separate from these two files are the actual extensions. Directories with names like this contain one extension each:


The name exactly matches the UUID that uniquely identifies the extension. Generation of UUIDs is done the same way as for XPCOM components [Hack #82] . The contents of these directories are the unpacked contents of the .xpi file in which the extension was delivered. Subdirectories named chrome, components, defaults, and uninstall inside each extension directory perform the same roles as those directories in the install area. For example, the .jar content files for the extension can be found in the extension's chrome subdirectory.

Firefox must scan all of these directories at startup (although the XUL FastLoad cache can reduce this burden to zero), so installing many extensions can affect startup performance a little. Then there is the matter of what the extensions do once they start up.

8.2.2. Spot Extension Installers

Extensions are delivered as .xpi (XPI stands for cross (X) platform install) files. They're usually delivered by clicking a link in an HTML page and require a special web server setting [Hack #27] . At the Mozilla Update web site (, you can see many of these files behind the download links, and if you context-click on one such link, you can save the .xpi file to disk and open it up instead of installing it. These files are ordinary ZIP files, except that they have a special directory layout that includes an optional install script named install.js and an install manifest named install.rdf. They can also be digitally signed.

8.2.3. Three Kinds of RDF Files

Here's a summary of the preceding remarks from a Resource Description Framework (RDF) perspective. RDF is used to provide all the management information about extensions, locales, themes, and packages.

Since extension content is typically delivered as one or more JAR files, extensions contain (inside those JAR files) one or more contents.rdf files. These files provide package registration and overlay registration information for the extension's chrome content. They're all about the lowest-level detailpackages and contentand they know nothing about extensions.

Extension JAR files are put into an XPI file along with an install.rdf file. This second RDF file contains all the registration information about the extension. The content of this file allows Firefox to manage the extension across versions, update it when required, and display its details in the Extension Manager dialog box.

Finally, the Extensions.rdf file in the profile area is the place where all of the information about extensions is aggregated. It contains all of the important information from all of the install.rdf files supplied by all of the extensions.

8.2.4. Compare Locales, Themes, Applications, and Extensions

Locales, themes, applications, and extensions are four types of downloadable enhancements that have a great deal in common. They all use some combination of content hierarchy that combines a package directory with one or more skin, locale, or content directories.

They differ in the registration information that's stored in RDF. Pieces of data for each kind of enhancement are recorded with different kinds of Uniform Resource Names (URNs). A URL can be either a URL or a URN; see the IETF RFC 2396 technical note for a definition ( Once you understand the basic URN naming scheme, browsing through the complex syntax of RDF files is a little easier.

These are the URN registration prefixes for themes and extensions:


These URNs are downloaded during extension or theme install. At the same time additional URNs that record the install bundle are also saved. Here's an example:


By comparison, locales are generally bundled with a localized Firefox install and are not installed via extensions. If they are delivered as extensions, they acquire the same registration prefixes as ordinary extensions do.

For install-bundled locales and install-bundled applications (like Firefox), the only registration URNs that appear are the traditional package ones:


This package configuration information is present in all cases, not just for bundled items.

8.2.5. Find the XPInstall Missing Link

When an extension is downloaded, it doesn't appear in the browser window as content and it doesn't get saved to disk (unless you choose that option). So how is it delivered to the Firefox profile area or install area?

The answer is XPInstall (Cross (X) Platform Install), a small piece of Firefox that offers a JavaScript hook in web pages and that can also recognize XPI content types. When an extension is installed, the XPI file is sucked directly into XPInstall (that is, into the Firefox executable's memory), where it is pulled apart. Each piece of the extension is then placed in the right spot. It's a system similar to InstallShield, but much less complex and entirely embedded inside the Firefox executable. XPInstall is close to fully automatic in its processing, so there's nothing much to do other than benefit from it.

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