For a while I was a bit confused, because Mozilla said they would also implement V3 Manifest …

by implementing Manifest V3 on its own terms, Mozilla saves developers who are switching to the new platform from having to support two different versions of their extensions (for Google Chrome and Firefox) at the same time. On the other hand, it allows content-blocking extensions that were originally built using the less restrictive Manifest V2 to continue working at full tilt.

https://adguard.com/en/blog/firefox-manifestv3-chrome-adblocking.html