I think you might be missing their concern. The concern is that google basically funds firefox, and can choose to revoke that funding at the most inconvenient

I think you might be missing their concern. The concern is that google basically funds firefox, and can choose to revoke that funding at the most inconvenient time for mozilla, risking bankruptcy. Companies ebb and flow on cash flow, and a unexpected drop at the exact wrong moment can cripple even the most well funded ones. Or just threaten to do so to exert pressure.