Unless you purchased some service from them, you didn't donate to Firefox, because that's a Mozilla Corporation project. Donating to Mozilla Foundation funds their initiatives, but not Firefox.
To Thunderbird, however, we can actually donate to.
At one point, paying for Mozilla's mullvad rebadge would give money to the corp. If you were already going to pay for a VPN, then it's effectively a donation.
Though, just because money goes to the corp, doesn't mean it will contribute to Firefox' development either.
To Thunderbird, however, we can actually donate to.