For the Dec 8 launch? I’d be more than happy to be wrong, but there’s just no way. As a diehard classic Firefight fan and as someone who has always generally preferred Halo’s PvE sandbox experience to the competitive MP scene, I’ve had my antennas tuned for any hint at a PvE experience beyond the campaign for this game for years, and I can’t recall any evidence whatsoever indicating that 343i has ever prioritized making a mode like this for launch.
Even if something has been in the works, the fact that features as core to Halo as Forge and Campaign Co-Op have been pushed to post-launch all but guarantees that we’re not getting something like a Firefight mode at launch, sadly.
That said, I do expect fully that 343i will attempt to recreate Firefight yet again and introduce it as a big Year 2 or 3 expansion. The only question is which, if any, legacy “Firefight” modes this new mode will resemble. At this point, let alone within the next handful of years, there are getting to be as many vocal fans of Warzone Firefight as there have been of ODST/Reach Firefight. Unfortunately, I think the design philosophies for these different modes aren’t really compatible, so 343i will have to “pick a lane” to a lesser or greater extent on that front.
Anyways, that last bit is all rampant speculation. All I’m super confident about is that Infinite’s main campaign will be the only PvE offering in this game for the foreseeable future. I wouldn’t expect to even hear about any additional PvE content via the inevitable campaign expansions until late next year.