Mozilla has revealed how it plans to adopt Manifest V3 in Firefox. The company says Firefox will fully embrace the next iteration of the web extension platform by the end of 2022, and in the lead-up to that, it's launching a new developer preview program to gather feedback from web developers.

What is Manifest V3?

Manifest V3 is the latest set of changes for the web extension platform to make extensions more secure, faster, and privacy-friendly. It was first announced by Google in 2019, and we have already discussed various big changes proposed in the new version and controversies surrounding it at great length. One of the most controversial changes in Manifest V3 is the removal of the Web Request API in favor of the new Declarative Net Request API. The developer community has widely criticized this change as it strips the extensions of many useful capabilities and makes them less effective. Google Chrome has already stopped accepting new extensions based on Manifest V2, while the existing Manifest V2 extensions will stop working after January 2023.

Mozilla wants to do things differently

Mozilla says it's aware of some of the controversial changes proposed in Manifest V3 and its crippling effect on ad-blockers and other extensions. To that end, Mozilla says it will take a different approach in implementing Manifest V3 than Google Chrome. Mozilla thinks that the new Declarative Net Request API isn't quite the perfect replacement for the WebRequest API, limiting the scope of ad blockers and privacy extensions. As such, Firefox will retain support for "blocking WebRequest" in Manifest V3 while also supporting DeclarativeNetRequest.

"Mozilla will maintain support for blocking WebRequest in MV3. To maximize compatibility with other browsers, we will also ship support for declarativeNetRequest. We will continue to work with content blockers and other key consumers of this API to identify current and future alternatives where appropriate," wrote Rob Wu, Senior Software Engineer, Mozilla.

Mozilla is also not a fan of Background Service Workers used by Chrome as it doesn't support many use cases and requires developers to rewrite a large part of extension code. Mozilla proposed Event Pages last year to address these shortcomings, which it says has been well received by the community and will be fully supported in Firefox's implementation of Manifest V3. Mozilla says it will also support Service Workers for compatibility reasons and because "we like that they're an event-driven environment with defined lifetimes, already part of the Web Platform with good cross-browser support."

"In Firefox, we have decided to support Event Pages in MV3, and our developer preview will not include Service Workers (we're continuing to work on supporting these for a future release). This will help developers to more easily migrate existing persistent background pages to support MV3 while retaining access to all of the DOM-related features available in MV2."

Source: Mozilla Blog