Setting Up An Update Server

The goal of this document is to provide instructions for installing a locally-served Firefox update for testing purposes. Note that these are not instructions for how to create or run a production update server. This method of serving updates is intended to trick Firefox into doing something that it normally wouldn’t do: download and install the same update over and over again. This is useful for testing but is obviously not the correct behavior for a production update server.

Obtaining an update MAR

Updates are served as MAR files. There are two common ways to obtain a MAR to use: download a prebuilt one, or build one yourself.

Downloading a MAR

Prebuilt Nightly MARs can be found here on Be sure that you use the one that matches your machine’s configuration. For example, if you want the Nightly MAR from 2019-09-17 for a 64 bit Windows machine, you probably want the MAR located at

Prebuilt MARs for release and beta can be found here. Beta builds are those with a b in the version string. After locating the desired version, the MARs will be in the update directory. You want to use the MAR labelled complete, not a partial MAR. Here is an example of an appropriate MAR file to use:

Building a MAR

Building a MAR locally is more complicated. Part of the problem is that MARs are signed by Mozilla and so you cannot really build an “official” MAR yourself. This is a security measure designed to prevent anyone from serving malicious updates. If you want to use a locally-built MAR, the copy of Firefox being updated will need to be built to allow un-signed MARs. See Building Firefox for more information on building Firefox locally. In order to use a locally built MAR, you will need to put this line in the mozconfig file in root of the build directory (create it if it does not exist):

ac_add_options --enable-unverified-updates

Firefox should otherwise be built normally. After building, you may want to copy the installation of Firefox elsewhere. If you update the installation without moving it, attempts at further incremental builds will not work properly, and a clobber will be needed when building next. To move the installation, first call ./mach package, then copy <obj dir>/dist/firefox elsewhere. The copied directory will be your install directory.

If you are running Windows and want the Mozilla Maintenance Service to be used, there are a few additional steps to be taken here. First, the maintenance service needs to be “installed”. Most likely, a different maintenance service is already installed, probably at C:\Program Files (x86)\Mozilla Maintenance Service\maintenanceservice.exe. Backup that file to another location and replace it with <obj dir>/dist/bin/maintenanceservice.exe. Don’t forget to restore the backup when you are done. Next, you will need to change the permissions on the Firefox install directory that you created. Both that directory and its parent directory should have permissions preventing the current user from writing to it.

Now that you have a build of Firefox capable of using a locally-built MAR, it’s time to build the MAR. First, build Firefox the way you want it to be after updating. If you want it to be the same before and after updating, this step is unnecessary and you can use the same build that you used to create the installation. Then run these commands, substituting <obj dir>, <MAR output path>, <version> and <channel> appropriately:

$ ./mach package
$ touch "<obj dir>/dist/firefox/precomplete"
$ MAR="<obj dir>/dist/host/bin/mar.exe" MOZ_PRODUCT_VERSION=<version> MAR_CHANNEL_ID=<channel> ./tools/update-packaging/ <MAR output path> "<obj dir>/dist/firefox"

For macOS you should use these commands:

$ ./mach package
$ touch "<obj dir>/dist/firefox/"
$ MAR="<obj dir>/dist/host/bin/mar.exe" MOZ_PRODUCT_VERSION=<version> MAR_CHANNEL_ID=<channel> ./tools/update-packaging/ <MAR output path> "<obj dir>/dist/firefox/"

For a local build, <channel> can be default, and <version> can be the value from browser/config/version.txt (or something arbitrarily large like 2000.0a1).

Note: It can be a bit tricky to get the script to accept paths with spaces.

Serving the update

Preparing the update files

First, create the directory that updates will be served from and put the MAR file in it. Then, create a file within called update.xml with these contents, replacing <mar name>, <hash> and <size> with the MAR’s filename, its sha512 hash, and its file size in bytes.

<?xml version="1.0" encoding="UTF-8"?>
    <update type="minor" displayVersion="2000.0a1" appVersion="2000.0a1" platformVersion="2000.0a1" buildID="21181002100236">
        <patch type="complete" URL="<mar name>" hashFunction="sha512" hashValue="<hash>" size="<size>"/>

If you’ve downloaded the MAR you’re using, you’ll find the sha512 value in a file called SHA512SUMS in the root of the release directory on for a release or beta build (you’ll have to search it for the file name of your MAR, since it includes the sha512 for every file that’s part of that release), and for a nightly build you’ll find a file with a .checksums extension adjacent to your MAR that contains that information (for instance, for the MAR file at, the file contains the sha512 for that file as well as for all the other win64 files that are part of that nightly release).

If you’ve built your own MAR, you can obtain its sha512 checksum by running the following command, which should work in Linux, macOS, or Windows in the MozillaBuild environment:

shasum --algorithm 512 <filename>

On Windows, you can get the exact file size in bytes for your MAR by right clicking on it in the file explorer and selecting Properties. You’ll find the correct size in bytes at the end of the line that begins “Size”, not the one that begins “Size on disk”. Be sure to remove the commas when you paste this number into the XML file.

On macOS, you can get the exact size of your MAR by running the command:

stat -f%z <filename>

Or on Linux, the same command would be:

stat --format "%s" <filename>

Starting your update server

Now, start an update server to serve the update files on port 8000. An easy way to do this is with Python. Remember to navigate to the correct directory before starting the server. This is the Python2 command:

$ python -m SimpleHTTPServer 8000

or, this is the Python3 command:

$ python3 -m http.server 8000

If you aren’t sure that you started the server correctly, try using a web browser to navigate to and make sure that you get the XML file you created earlier.

Installing the update

You may want to start by deleting any pending updates to ensure that no previously found updates interfere with installing the desired update. You can use this command with Firefox’s browser console to determine the update directory:

ChromeUtils.importESModule("resource://gre/modules/FileUtils.sys.mjs").FileUtils.getDir("UpdRootD", []).path

Once you have determined the update directory, close Firefox, browse to the directory and remove the subdirectory called updates.

Next, you need to change the update URL to point to the local XML file. This can be done most reliably with an enterprise policy. The policy file location depends on the operating system you are using.
Windows/Linux: <install dir>/distribution/policies.json
macOS: <install dir>/Contents/Resources/distribution/policies.json
Create the distribution directory, if necessary, and put this in policies.json:
  "policies": {
    "AppUpdateURL": ""

Now you are ready to update! Launch Firefox out of its installation directory and navigate to the Update section about:preferences. You should see it downloading the update to the update directory. Since the transfer is entirely local this should finish quickly, and a “Restart to Update” button should appear. Click it to restart and apply the update.