PrefPicker

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Manage & generate prefs.js files for use when testing Firefox. This tool is intended to simplify the use and tracking of prefs used by our fuzzing tools. The template files can be modified to allow the creation of custom prefs.js files without the need to maintain a separate mostly duplicate version of a prefs file.

YAML Template Structure

The template document is made up of variants, prefs and values.

pref is the name of the preference that will be added to the prefs.js file. This is an unquoted string. Valid prefs can be found in all.js or in StaticPrefList.yml.

review_on_close is optional. It is a list of relevant Bugzilla IDs used to help avoid obsolete entries. When all bugs in the list are closed the entry will be reviewed and removed if appropriate.

value can be a bool, int, string or null. Adding multiple potential values is possible. When multiple values are present one is chosen at random when generating the output. Using a value of null will exclude the pref from the prefs.js file (acts as browser default).

variant is a subset of values to be used in place of the default values. The default variant is used unless a variant is specified.

There are a few mechanisms in place to help keep the file in order:

  • All prefs must have a default variant
  • All variants must be defined in the variant list
  • All variants in the variant list must be used
  • All variants must be a list and contain values
# example.yml
variant:              # list of extra variants, default is implied
- alt                 # name of variant
pref:
  pref.name:          # unquoted name of the pref used in prefs.js
    review_on_close:  # optional
    - 123456
    variants:
      default:        # variant definition, default is required
      - 0             # potential value
      alt:            # extra optional variant
      - 1             # if multiple values are defined one is chosen randomly
      - null          # null is a special case meaning exclude the pref

Updating Templates and Adding Prefs

Prefs are found in the .yml files in the template directory. Only prefs that are ready to be tested should be added. When adding a pref to a template it is encouraged to add a comment that provides justification and points to a bug in Bugzilla for additional context. If a pref does not already exist and is only used with non-default variants a null entry must be added to the default variant.

Quick Setup

Use pip to install prefpicker.

pip install prefpicker

Examples

Use a built-in template to generate an up-to-date prefs.js file.

prefpicker browser-fuzzing.yml prefs.js

Or generate a prefs.js file from a custom template using the webrender variant:

user@machine:~/prefpicker$ prefpicker custom/template.yml ~/Desktop/prefs.js --variant webrender
Loading 'template.yml'...
Loaded 255 prefs and 5 variants
Generating 'prefs.js' using variant 'webrender'...
Done.

The resulting prefs.js file is ready to be used with Firefox. It will look something like this:

// Generated with PrefPicker @ 2020-02-08 00:50:29 UTC
// Variant 'webrender'
/// ... snip
user_pref("fuzzing.enabled", true);
/// ... snip
// 'gfx.webrender.all' defined by variant 'webrender'
user_pref("gfx.webrender.all", true);
/// ... snip

Project links

Data verified by PyPI on Aug 20, 2026
Data provided by the project maintainers, verified at the time the release was uploaded to PyPI.

Key dates

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  • Released:
Newer release available (2.27.0)

2 maintainers

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Avatar for mozillasecurity from gravatar.com mozillasecurity Avatar for twsmith from gravatar.com twsmith

Credits

Author: Tyson Smith

Maintainer: Mozilla Fuzzing Team

GitHub Statistics

Data verified by PyPI on Aug 20, 2026
The GitHub source repository was provided by the project maintainers and verified by PyPI at the time of upload. Stars, forks, and open issues/PRs are derived from that repository and have not been independently verified.

License

Mozilla Public License 2.0 (MPL 2.0) (MPL 2.0)

Requires

Python >=3.10

Provides Extra

dev

Tags

firefox fuzz fuzzing test testing

Classifiers

Report project as malware

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