Technology

Not having control of the core codebase, and branching/tracking based on 1 (declared) legacy feature could lead to huge amounts of work and issue in the future.
Manifest V2 spec is defined, manifest V3 spec is defined... They can be developed against.
JS-whatever-spec is defined, CSS-whatever-spec is defined, HTML-whatever-spec is defined... They have industry standard approved specs (even if they can be vague in areas). They can be developed against.
They have defined spec documents that can be developed against.

Firefox has control and experience of how they implement those specs.
Chrome forks do not have control of how those specs are implemented.
So if chrome changes how things are implemented, forks might not be able to "backport" for manifest V2 compatibility, and might find themselves implementing more and more of the core browser functionality. Browsers are NOT easy to develop for the modern fuckery of the web.
Firefox hopefully does have that knowledge and ability to include V2 manifest backwards compatibility in future development without impacting further spec implementations.... It seems like Google is depreciating V2 to combat ad-blockers (ads being their major funding revenue)

There are already very slight differences how Firefox and Chrome interpret all these specs. I've noticed a few sites & plugins that just work better (or just work) in Chrome. Which is why I still have (unfortunately) an install of Chrome.