This is not a mobile first project, it's one written to be responsive and compatible with both desktop and mobile.
The design ethos of Flarum is always to be a simple forum framework, and let users extend it as they need. In fact, many of the major components of Flarum, the tags system, the likes and mentions, are all extensions that can be enabled and disabled based on the site admin's preference. So any editor extension, whether one provided by the community (of which there are several already) or by Flarum in the future, could be enabled or not as a site admin decided.
That also means that any extension Flarum may create can be rewritten by the community for different needs. So there's really no danger of the mobile experience being compromised, features can be adjusted on a per-site basis by using (or not using) extensions.
