Unless it's literally no effort to maintain extensions that use both, a large portion of maintainers will develop what has the largest market share. Sure for uBlock Origin, there's enough momentum to maintain a v2 version for Firefox, but for a new extension with one developer, it's unlikely that they'd make two versions.
Either this backfires, and Firefox ends up having the better extensions using v2 manifest, or new extensions will be developed with the limitations of v3 and Firefox users will have an unnecessarily neutered experience as Chrome users.
I think Firefox will support both v2 and v3 extensions, so devs can use whichever makes more sense for their project. It has been a while since I looked into it though.