My understanding if you given money to Mozilla who then give it to mullvad

All Mozilla know is the mullvad username. Mullvad don’t know the credit card details of the purchaser. To link a given vpn ip to a specific credit card would require compromising mullvad and Mozilla.

(Or of course the normal way of fingerprinting which doesn’t rely on IPs)

Now sure you can buy mullvad via cash, but that’s far more work. Using Mozilla as a reseller feels like one more step in the chain