I was told I requested the sign-up code too fast. IMHO they’re receiving too many signups at once and they’re being rate-limited by their SMS provider…
Try insync (one-time payment), and works like a charm. I've been using it for a couple of years, zero problems. Syncs noy only OneDrive, also Google Drive and others.
Mozilla, you are still on time to reroute your efforts to regain the users you'd lost by crap like this.