Firefox will support Manifest V3 extension specification
However, this specification is controversial because it affects ad blockers. Google believes that the webRequest API provides “access to potentially sensitive user data”, but is usually used by adblockers, so it uses the declarativeNetRequest API instead. In this regard, Mozilla stated that it will continue to allow the previous methods, but will provide support for the declareNetRequest API so that developers can choose.
In addition, Mozilla agrees with Google’s decision to ensure that the extension basically does not keep a full-page open in the background in order to run. Firefox will support service workers to handle background tasks and events. Mozilla will also implement cross-source protection to help protect cookie privacy and allow users to control which website extensions can be activated.
It is reported that Firefox will begin to let developers test the Manifest V3 extension specification in the fourth quarter of 2021, and will officially accept the new extension early next year. However, this schedule may be pushed back or delayed due to unforeseeable circumstances.