Firefox plans to support Manifest V3 because Chrome is the world’s most popular browser, and it wants extensions to be cross-browser compatible, but it has no plans to turn off support for Manifest V2.
If Google decided to break V2 compatibility with V3, Mozilla should announce V4 (or V3 extended), which is V3 but with the missing stuff readded.
That’d be a good practical and great product/tech marketing move. Just like most people won’t see how V3 is worse than V2, V4 will indicate it’s the evolved and improved V3.
It would also simplify supporting V3 and V4 at the same time for extension authors. A great practical gain for extension authors, not having to read and understand two manifest schemes and APIs.