JAR Manifests

JAR Manifests are plaintext files in the tree that are used to package chrome files into .jar files and create Chrome Registration manifests. JAR Manifests are commonly named jar.mn. They are declared in moz.build files using the JAR_MANIFESTS variable, which makes up a collection of jar.mn files. All files declared in JAR Manifests are processed and installed into omni.ja files in browser/ and toolkit/ when building Firefox.

jar.mn files are automatically processed by the build system when building a source directory that contains one. The jar.mn is run through the Text Preprocessor before being passed to the manifest processor. In order to have @variables@ expanded (such as @AB_CD@) throughout the file, add the line #filter substitution at the top of your jar.mn file.

The format of a jar.mn is fairly simple; it consists of a heading specifying which JAR file is being packaged, followed by indented lines listing files and chrome registration instructions.

For a simple jar.mn file, see toolkit/profile/jar.mn. For a much more complex jar.mn file, see toolkit/locales/jar.mn. More examples with specific formats and uses are available below.

Shipping Chrome Files

General Format

To ship chrome files in a JAR, an indented line indicates a file to be packaged:

  path/in/jar/file_name.xul     (source/tree/location/file_name.xul)

Note that file path mappings are listed by destination (left) followed by source (right).

Same Directory Omission

If the JAR manifest and packaged files live in the same directory, the source path and parentheses can be omitted. A sample of a jar.mn file with omitted source paths and parentheses is this revision of browser/components/colorways/jar.mn:


Writing the following is equivalent, given that the aforementioned files exist in the same directory as the jar.mn. Notice the .jar file is named browser.jar:

  content/browser/colorwaycloset.html (colorwaycloset.html)
  content/browser/colorwaycloset.css  (colorwaycloset.css)
  content/browser/colorwaycloset.js   (colorwaycloset.js)

This manifest is responsible for packaging files needed by Colorway Closet, including JS scripts, localization files, images (ex. PNGs, AVIFs), and CSS styling. Look at browser/components/colorways/colorwaycloset.html to see how a file may be referenced using its chrome URL.

Absolute Paths

The source tree location may also be an absolute path (taken from the top of the source tree). One such example can be found in toolkit/components/pictureinpicture/jar.mn:

  * content/global/pictureinpicture/player.xhtml   (content/player.xhtml)
  content/global/pictureinpicture/player.js      (content/player.js)

Asterisk Marker (Preprocessing)

An asterisk marker (*) at the beginning of the line indicates that the file should be processed by the Text Preprocessor before being packaged. The file toolkit/profile/jar.mn indicates that the file toolkit/profile/content/profileDowngrade.xhtml should be run through the preprocessor, since it contains #ifdef and #endif statements that need to be interpreted:

* content/mozapps/profile/profileDowngrade.xhtml  (content/profileDowngrade.xhtml)

Base Path, Variables, Wildcards and Localized Files

The .jar file location may be preceded with a base path between square brackets. The file toolkit/locales/jar.mn uses a base path so that the .jar file is under a localization directory, which is a special directory parsed by mozbuild.

It is also named according to the value passed by the variable @AB_CD@, normally a locale. Note the use of the preprocessor directive #filter substitution at the top of the file for replacing the variable with the value:

#filter substitution


[localization] @AB_CD@.jar:
  crashreporter                                    (%crashreporter/**/*.ftl)
  toolkit                                          (%toolkit/**/*.ftl)

The percentage sign in front of the source paths designates the locale to target as a source. By default, this is en-US. With this specific example, /toolkit/locales/en-US would be targeted. Otherwise, the file from an alternate localization source tree /l10n/<locale>/toolkit/ is read if building a localized version. The wildcards in **/*.ftl tell the processor to install all Fluent files within the crashreporter and toolkit directories, as well as their subdirectories.

Registering Chrome

Chrome Registration instructions are marked with a percent sign (%) at the beginning of the line, and must be part of the definition of a JAR file. Any additional percents signs are replaced with an appropriate relative URL of the JAR file being packaged.

There are two possible locations for a manifest file. If the chrome is being built into a standalone application, the jar.mn processor creates a <jarfilename>.manifest next to the JAR file itself. This is the default behavior.

If the moz.build specifies USE_EXTENSION_MANIFEST = 1, the jar.mn processor creates a single chrome.manifest file suitable for registering chrome as an extension.


The file browser/themes/addons/jar.mn registers a resource chrome package under the name builtin-themes. Its source files are in %content/builtin-themes/:

  %  resource builtin-themes %content/builtin-themes/

  content/builtin-themes/alpenglow                 (alpenglow/*.svg)
  content/builtin-themes/alpenglow/manifest.json   (alpenglow/manifest.json)

Notice how other files declare an installation destination using the builtin-themes resource that is defined. As such, a SVG file preview.svg for a theme Alpenglow may be loaded using the resource URL resource://builtin-themes/alpenglow/preview.svg so that a preview of the theme is available on about:addons. See Chrome Registration for more details on resource and other manifest instructions.