Firefox will also switch to manifest v3, although Mozilla claims they will use non-inferior api. Only Brave may never be affected, because it uses a native built-in ad blocker.
And I doubt it will happen but if by some miracle people were switching to Firefox due to v2 phase out in chrome it might send a strong signal to keep supporting it.
Firefox's Manifest V3 implementation arguably isn't ready for prime time (it has some pretty egregious bugs), but whenever they finally do get around to dropping Manifest V2 (which I have no idea when that would happen), I'm not really worried about extensions not being able to jump over.