AFAIK, the foundation only does "other stuff", and the development is fully funded by the corporation (i.e. Google), so you can't really sponsor Firefox develop

AFAIK, the foundation only does "other stuff", and the development is fully funded by the corporation (i.e. Google), so you can't really sponsor Firefox development at all, except by buying unrelated services like Pocket and the VPN (and who knows where your money goes then).