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Desktop Migration Guide (JS)

To migrate front-end normandy prefs to Nimbus controlled experiments or rollouts

This guide will help you migrate your Desktop front-end code to run experiments with Nimbus, while still being able to use preferences for default and user-override values


  • Your experimental variables are already instrumented with Firefox preferences
  • You don't use the user branch of each pref for anything other than actual user-defined values or testing (see docs on order of precedence
  • Your code can import a jsm

An illustrative example (about:myself)

For the purposes of this guide, we will be migrating an imaginary about page (about:myself), which uses the following preferences defined in firefox.js:

pref("browser.aboutmyself.enabled", true);
pref("browser.aboutmyself.bgcolor", "#FE8DAE");

Step 1: Add a new feature to the manifest

First, you will need to register a new feature in FeatureManifest.yaml. In this case, we're creating one called aboutmyself. After adding the feature a build step is required to update the appropriate header file.

Read more to find out if you want to send an exposure event. This is optional but a decision must be recorded in the manifest.

Each preference is registered as a variable:

description: A page that shows personal browsing stats
# Exposure is optional, in which case `hasExposure` would be false
# and `exposureDescription` would not be defined
hasExposure: true
exposureDescription: The exposure is the earliest moment that the user could be affected by the experimental treatment
type: boolean
fallbackPref: browser.aboutmyself.enabled
type: string
fallbackPref: browser.aboutmyself.bgcolor

Step 2: Update your feature code

First, you will need to import ExperimentAPI.jsm:

XPCOMUtils.defineLazyModuleGetters(this, {
NimbusFeatures: "resource://nimbus/ExperimentAPI.jsm",

Then anywhere in your code that uses Services.prefs to get experiment-defined values, use NimbusFeatures instead: = Services.prefs.getBoolPref("browser.aboutmyself.bgColor");

becomes = NimbusFeatures.aboutmyself.getVariable("bgColor");

See the API reference docs for more details, including listening to changes.

Step 3: Run tests

If you've configured fallback preferences your tests should pass as written, but we recommend also reading Testing Guide to add experiment-specific tests.


What happens if I run a Normandy experiment and a Nimbus experiment simultaneously?

If possible, we do not recommend you run Normandy and Nimbus experiments on the same user population simultaneously. If a user were to enroll in an experiment from both sources, the value returned would follow the rules defined in Configuration Sources docs. (Nimbus values override default pref branch values; user pref branch values override Nimbus).

If you have a long-running Normandy experiment during which you must launch a Nimbus experiment, you can add a custom targeting rule to exclude users from a specific experiment ID.