While MV3 is still in development, many major features are already included in the Developer Preview, which provides an opportunity to expose functionality for testing and feedback. With strong developer feedback, we’re better equipped to quickly address critical bug fixes, provide clear developer documentation, and reorient functionality.
Some features, such as a well defined and documented lifecycle for Event Pages, are still works in progress. As we complete features, they’ll land in future versions of Firefox and you’ll be able to test and progress your extensions into MV3 compatibility. In most ways Firefox is committed to MV3 cross browser compatibility. However in some cases Firefox will offer distinct extension functionality.
Developer Preview is not available to regular users; it requires you to change preferences in about:config. Thus you will not be able to upload MV3 extensions to addons.mozilla.org (AMO) until we have an official release available to users.
The following are key considerations about migration at this time and areas we’d greatly appreciate developer feedback.
- Read the MV3 migration guide. MV3 contains many changes and our migration guide covers the major necessary steps, as well as linking to documentation to help understand further details.
- Update your extension to be compatible with Event Pages. One major difference in Firefox is our use of Event Pages, which provides an alternative to the existing Background Pages that allows idle timeouts and page restarts. This adds resilience to the background, which is necessary for resource constraints and mobile devices. For the most part, Event Pages are compatible with existing Background Pages, requiring only minor changes. We plan to release Event Pages for MV2 in an upcoming Firefox release, so preparation to use Event Pages can be included in MV2 addons soon. Many extensions may not need all the capabilities available in Event Pages. The background scripts are easily transferable to the Service Worker background when it becomes available in a future release. In the meantime, extensions attempting to support both Chrome and Firefox can take advantage of Event Pages in Firefox.
- Test your content scripts with MV3. There are multiple changes that will impact content scripts, ranging from tighter restrictions on CORS, CSP, remote code execution, and more. Not all extensions will run into issues in these cases, and some may only require minor modifications that will likely work within MV2 as well.
- Understand and consider your migration path for APIs that have changed or deprecated. Deprecated APIs will require code changes to utilize alternate or new APIs. Examples include New Scripting API (which will be part of MV2 in a future release), changing page and browser actions to the action API, etc.
- Test and plan migration for permissions. Most permissions are already available as optional permissions in MV2. With MV3, we’re making host permissions optional — in many cases by default. While we do not yet have the primary UI for user control in Developer Preview, developers should understand how these changes will affect their extensions.
- Let us know how it’s going! Your feedback will help us make the transition from MV2 to MV3 as smooth as possible. Through Developer Preview we anticipate learning about MV3 rough edges, documentation needs, new features to be fleshed out, and bugs to be fixed. We have a host of community channels you can access to ask questions, help others, report problems, or whatever else you desire to communicate as it relates to the MV3 migration process.
Stay in touch with us on any of these forums…
- Online chat: chat.mozilla.org
- Report issues and ask questions on Discourse (where knowledge may be shared among other developers)
- Report problems on: Bugzilla
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