MSIX Package

Firefox MSIX packages are full participants in the “modern” Windows app packaging system. They are distributed, installed, updated, repaired, and uninstalled entirely using that system. This gives administrators lots of deployment options, and also grants complete control over when and how application updates are rolled out (Firefox’s built-in updater is always fully disabled in MSIX packages). This stands in contrast to Firefox MSI packages, which mostly work against the Windows Installer framework rather than with it, and therefore are missing a lot of important functionality; for example, tools that install MSI packages generally cannot uninstall Firefox [1]. This means the MSIX package may well be the better option for deploying to Windows 10 and up.

In automation

The repackage-msix and repackage-shippable-l10n-msix tasks repackage the ZIP packages produced by signed build tasks into MSIX packages. The shippable-l10n variants depend on Linux64 builds and localization tasks to produce signed langpacks, which are incorporated into the MSIX package as distribution extensions. (This is the same approach taken by snap and flatpak Linux packages.)

The repackage-signing-msix and repackage-signing-shippable-l10n-msix tasks sign the MSIX packages produced by the repackage tasks.

Signing in automation

MSIX packages are signed by the same certificates that sign binaries for other jobs. In practice, this means that try builds are signed with the `Mozilla Fake CA certificate MozFakeCA_2017-10-13.cer. In order to install try builds locally, you must trust this certificate. For your own security, it’s best to do this in Windows Sandbox or a Virtual Machine. To do so run the following in an elevated (administrator) Powershell:

$ Import-Certificate -FilePath "MozFakeCA_2017-10-13.cer" -Cert Cert:\LocalMachine\Root\
Thumbprint                                Subject
----------                                -------
FA056CEBEFF3B1D0500A1FB37C2BD2F9CE4FB5D8  CN=Mozilla Fake CA

The shippable-l10n MSIX variants incorporate signed langpacks. These in turn are signed with the same certificate. Firefox knows about this certificate but does not trust it by default. To trust it, set the hidden Gecko boolean preference

Sadly, it’s not possible to set preferences via a distribution.ini early enough to impact loading the signed langpacks (see Bug 1721764), and once the signed langpacks fail to load once, they will not be reloaded (see Bug 1721763). This make testing the first-run experience challenging. What can be done is to install the MSIX package (perhaps using Add-AppxPackage -Path ...) and determine the profile directory (using about:support). Uninstall the MSIX package (perhaps using Get-AppxPackage | Where -Property Name -like "Mozilla.*" | Remove-AppxPackage). Delete the contents of the profile directory entirely, but add a file user.js containing:

user_pref("", true);
user_pref("extensions.logging.enabled", true);

Reinstall the MSIX package and the signed langpacks should now be loaded (slowly!) and available after first startup.

Local developer builds

mach repackage msix lets you repackage a Firefox package (or directory) into an MSIX/App Package. The main complication is that an App Package contains channel-specific paths and assets, and therefore needs to be branding-aware, much as an Android package needs to be branding-aware.


The tool is designed to repackage ZIP archives produced in automation. Start looking for official builds at locations like:







Repackage using commands like:

$ ./mach repackage msix \
  --input \
  --channel=official \
  --arch=x86_64 \

Or package a local developer build directly with mach repackage msix:

$ ./mach repackage msix

This command will do its best to guess your channel and other necessary information. You can override these with options like --channel (see the --help text for all supported options).

Paths to tools can be set via environment variables. In order, searched first to searched last:

  1. the tool name, like MAKEAPPX or SIGNTOOL

  2. searching on PATH

  3. searching under WINDOWSSDKDIR

  4. searching under C:/Program Files (x86)/Windows Kits/10

If you are cross compiling from Linux or macOS you will need a compiled version of Mozilla’s fork of Microsoft’s msix-packaging tools.

Linux users can obtain a prebuilt version with:

$ ./mach artifact toolchain --from-build linux64-msix-packaging

After bug 1743036 is fixed, macOS and Windows users will have a similar option.

Avoiding signing locally with unsigned packages

Windows 11 allows to install unsigned packages, provided that its AppX manifest includes a special OID (organization ID) value in its Identity element. See To produce a suitable package, use the --unsigned command line switch, like:

$ ./mach repackage msix --unsigned

Note that unsigned packages must be installed by an administrator. Generally, run Powershell as an administrator and then use commands like

$ Add-AppxPackage -Path ... -AllowUnsigned -ForceUpdateFromAnyVersion

Signing locally

The repackaged MSIX files produced are not signed by default. In automation, Mozilla’s signing service signs the repackaged MSIX files. For local testing, you can sign them with a self-signed certificate by adding --sign to mach repackage msix, or with commands like:

$ ./mach repackage sign-msix --input test.msix --verbose

Or sign them yourself following Microsoft’s self-signed certificate instructions.

Signing Certificates

Mach will create the necessary signing keys and certificates for you and reuse them for subsequent signings. Before your locally signed builds can be installed you will need to install the correct certificate to the Windows Root Store. This can be done with a command like:

$ powershell -c 'Import-Certificate -FilePath mycert.cer -Cert Cert:\LocalMachine\Root\'

The exact command to run will be shown if you run ./mach repackage with --verbose.

You _may_ choose to sign in a different manner, with a key and certificate you create yourself, but Windows requires that the Subject of the certificate match the Publisher found in the MSIX’s AppxManifest.xml. If you choose to go this route, ensure that you pass --publisher to ./mach repackage msix to set that correctly.

For developers

Updating the MSIX template

MSIX is an “open format” in one sense: the MSIX container format is specified at It is categorically not an open format in another sense: many of the contained files are proprietary binary formats (.reg – registry hive files) or undocumented (.pri files – resource index files).

Generally the MSIX packaging code tries to avoid requiring such files that can’t be built from sources. Where they are truly required, it tries to use a single such file independent of branding and other configuration, checked into the source tree.


Generate a new resources.pri file on a Windows machine using makepri.exe from the Windows SDK, like:

$ makepri.exe new \
    -IndexName firefox \
    -ConfigXml browser/installer/windows/msix/priconfig.xml \
    -ProjectRoot browser/branding/official/msix \
    -OutputFile browser/installer/windows/msix/resources.pri \

The choice of channel (i.e., browser/branding/{official,aurora,beta,nightly,unofficial}) should not matter.