Review Policies

In order to protect the safety and sovereignty of Firefox users, Mozilla requires all add-ons to comply with a set of policies on acceptable practices. The exact set of applicable policies varies depending on a number of circumstances, the most important being whether the add-on is hosted on addons.mozilla.org (hereafter AMO), and how the add-on is distributed in the wild.

This document outlines the policies which different classes of add-on are expected to obey. Regardless of the class of add-on, these policies are enforced via a mandatory review process facilitated by AMO, and a mandatory code signature check, enforced by Firefox.

Add-on Classes

Listed on AMO

Add-ons listed on AMO must undergo review by a human reviewer. Prior to review, add-ons will be accessible to users who have a direct link to their listing pages, but are otherwise hidden from the public. Once approved, these add-ons will have a public listing page including screenshots, descriptions, and user reviews. The listing will appear in search results, collections, and occasional promotions. Existing users will automatically receive updates to new versions published to AMO.

There are two classes of review for add-ons listed on AMO, each with its own stringency of requirements and feature set:

Full Review: Add-ons in the full review class undergo full code review, as well as functional testing. These add-ons are subjected to the highest quality bar. In exchange are given the highest precedence in search results, and present the most streamlined install experience to users.
Preliminary Review: Add-ons in the preliminary review class undergo full, though less detailed, code review, but in general do not undergo functional testing. These add-ons must not cause security problems, or seriously hamper the usability of the browser, but otherwise have few qualifications. As a result, these add-ons are displayed less prominently in search results, and their listing pages will warn users about potential quality issues prior to install.

Unlisted

Unlisted add-ons must be uploaded to AMO prior to distribution, but are otherwise not accessible to the public via the site. These add-ons must be distributed elsewhere by their publishers. Depending on the manner of distribution, unlisted add-ons will undergo a fully-automated review, with possible post-signing code reviews.

There are two classes of unlisted add-ons:

Side-install: This review class, sometimes referred to instead as "full review", is required for any add-ons which will be installed into the browser by application installers, rather than directly by users via the web install process. While these add-ons are automatically signed, they are held to a very similar standards to those of listed add-ons in the full review track. The primary difference is that these add-ons must manage their own updates.
Web Install: This review class, sometimes referred to instead as "preliminary review", is for add-ons distributed entirely via web install, initiated directly by the user. Add-ons in this class have the least stringent requirements.

Policies

The following table outlines the primary policies which apply to each add-on review track. The policies are explained in further detail below. The symbols below each review track specify how the requirement applies to those add-ons as follows:

	The requirement does not apply to this review track.
✘	Add-ons in this review track are prohibited from engaging in this behavior.
✔	Add-ons in this review track must follow this behavior.

	Listed		Unlisted
	Full	Prelim	Side-install	Web Install
Security
Cause harm to users' data, systems, or online identities	✘	✘	✘	✘
Create or expose security vulnerabilities	✘	✘	✘	✘
Tamper with the application/add-on update or blocklist systems	✘	✘	✘	✘
Execute remote code¹	✘	✘	✘
Degrade the security of HTTPS sites	✘	✘	✘
Install additional add-ons or system applications without user consent	✘	✘
Include their own update mechanism	✘	✘
Privacy and User Consent
Make unexpected changes to the browser or web content	✘	✘	✘	✘
Prevent users from reverting changes made by the add-on	✘	✘	✘	✘
Prevent the add-on from appearing in the Add-on Manager	✘	✘	✘	✘
Prevent the user from disabling or uninstalling the add-on⁶	✘	✘	✘	✘
Send sensitive information to remote servers unprotected	✘	✘	✘
Store browsing data from private browsing windows	✘	✘	✘
Leak identity information to web content in private browsing windows	✘	✘	✘
Change Firefox preferences without user consent	✘		✘
Clearly disclose all user data handling in a Privacy Policy	✔		✔
User Experience
Break or disable core application features	✘	✘	✘	✘
Make any changes which persist after the add-on is disabled or uninstalled	✘	✘	✘	✘
Be easy to use and provide a consistent user experience	✔		✔
Appeal to a general audience	✔
Content
Violate the Mozilla acceptable use policy	✘	✘	✘
Technical
Provide reviewers with full source code⁵	✔	✔	✔	✔²
Use unvetted third-party code libraries or frameworks	✘	✘	✘
Contain obvious coding errors	✘		✘
Conflict with other well-behaved add-ons³	✘		✘
Use APIs known to cause performance or stability problems⁴	✘		✘

¹ Remote code may be executed in documents with the same origin as the code being executed, or, under limited circumstances, in carefully constructed sandboxes. Remote code may never be executed in privileged contexts.

² Unlisted add-ons must provide sources upon request. Failure to provide sources for an automatically signed add-on upon explicit request may lead to the add-on being blocked.

³ It may not be possible for all add-ons to entirely avoid conflict with all other add-ons. Add-ons which, by nature, cannot operate side-by-side may be allowed to conflict. Conflicts due to poor technical practices will not be tolerated.

⁴ APIs which have been deprecated for performance or stability reasons, including DOM mutation event listeners, synchronous XMLHttpRequests and Storage API calls, and code which re-enters the main event loop, should not be used in add-ons. They may be allowed under limited circumstances, where alternatives are impractical, or be granted a reimplementation grace period, but such exceptions are rare, and in general the APIs should be avoided as a mater of course.

⁵ Separate submission of full source code is required for add-ons which use obfuscation, minification, or transcompilation to generate JavaScript source code, or which include executable binary files, including system executables or libraries. Instructions and tools necessary to reproduce obfuscation may also be required. Add-ons which include only human-readable JavaScript are not required to submit separate source code.

⁶ Users must be able to disable and install the add-on via the Add-on Manager interface. Providing secondary methods of uninstall, such as a system-level uninstaller, while preventing it via the Add-on Manager interface, does not satisfy this requirement.

Security

Because add-ons run in an environment with elevated privileges relative to ordinary web pages, they present a very serious set of security considerations. The have the potential to open up security holes not only in the add-ons themselves, but also in the browser, in web pages, and in particularly worrying cases, the entire system the browser is running on. As a result, we take our security policies very seriously, and apply most of them to all add-ons, whether hosted on AMO or not. We expect all add-ons to be secure, not only in their handling of their own data, and of user data, but also in all of their interaction with the web, the browser, and the operating system.

Privacy and User Consent

We take user sovereignty and privacy extremely seriously. Whether hosted on AMO or not, we require all add-ons to respect users choices and their reasonable expectations of privacy. In particular, this means that add-ons may not limit users control of their browsers, by making it impossible to permanently change settings (such as the homepage or search engine), preventing users from uninstalling them, hiding their presence from users, or installing toolbar buttons or other UI elements which cannot be permanently removed via the UI customization process.

Features like advertising or certain forms of user activity tracking may be required to be opt-in, or at least opt-out, depending on the privacy and security impact, and whether the feature is necessary for the add-on to function or not. Since these are usually additional monetization features that are unrelated to what the add-on is meant to do, they generally require an opt-in for listed add-ons and an opt-out for unlisted ones. Some forms of tracking, like gathering all visited URLs, are generally forbidden even for unlisted add-ons. The decision to activate or deactivate these features and its implications must be clearly presented to the user.

User Experience

We expect all add-ons to work without significantly degrading users' experience with the browser. In particular, add-ons may not adversely affect browser performance, break built-in features, or damage the user interface. For add-ons listed on AMO, requesting full review, we likewise expect a consistent generally positive user experience for any functionality provided by the add-on.

Content

While we have no interest in controlling the types of functionality provided by add-ons in the wild, there are certain types of content that addons.mozilla.org cannot host. In particular, all content hosted on the site must conform to the laws of the United States, and comply with the Mozilla acceptable use policy.

Technical

We try, as much as possible, not to restrict the freedom of developers to maintain their add-ons as they choose. However, for reasons of security and our ability to effectively review code, we do have certain technical requirements. In particular, potentially dangerous APIs, such as those which evaluate HTML or JavaScript, may only be used in ways which are provably safe, and code which we cannot verify to behave safely and correctly may need to be refactored.

Source Code Submission

Add-ons may contain binary, obfuscated and minified source code, but Mozilla must be allowed to review a copy of the human-readable source code of each version of an add-on submitted for review. In such cases, the author will receive a message when the add-on is reviewed indicating whom to contact at Mozilla to coordinate review of the source code. This code will be reviewed by an administrator and will not be shared or redistributed in any way. The code will only be used for the purpose of reviewing the add-on.

If your add-on contains code that you don't own or can't get the source code for, you may contact us for information on how to proceed.

Reviewers

Add-ons are reviewed by the AMO Reviewer Team, a group of experienced add-on developers that volunteer to help the Mozilla project by reviewing add-ons to ensure a stable and safe experience for users. The Reviewer Guide details how reviewers evaluate add-ons submitted for review. It is an expanded version of the table shown above. Developers will receive an email with any updates throughout the review process. Review times can fluctuate depending on reviewer availability and the complexity of the add-on being reviewed. Regular updates of review queue status are posted in the Add-ons Blog.

Blocklisting

Add-ons that don't meet the bar for Unlisted Web Install may qualify for blocklisting, depending on the extent of their problems. The Add-ons Team will do their best to contact the add-on's developers and provide a reasonable time frame for the problems to be corrected before a block is deployed. If an add-on is considered malicious or its developers have proven unreachable or unresponsive, or in case of repeat violations, blocklisting may be immediate.

Guideline violations should be reported via Bugzilla, under Tech Evangelism > Add-ons. Questions can be posted in the #addons IRC channel.