I’m a Mozilla employee who works on Firefox, so I’ll try to answer this to the best of my knowledge but as a disclaimer I can’t guarantee I’m 100% correct

Paying for relay will give money to Mozilla Corporation, the same pot the google money goes into, which will predominantly pay for Firefox development but also other products. The corporation’s profits also fund the non-profit Foundation’s activities.

People often raise this argument regarding donating to the Foundation, as that money will be spent by the foundation, therefore not on Firefox. But a dollar raised by the foundation is a dollar less the corporation has to give the foundation, leaving it with more money to spend on Firefox and other things.

You can also donate directly to “MZLA” which makes thunderbird, and that money will be spent on thunderbird.

The problem many people have with donating to the Mozilla Foundation us them squandering enormous amounts of it. Mostly on things nobody asked for and executive pay.

Personally, I don’t feel like firing Firefox devs and starting controversial and expensive diversity campaigns while raising executive pay when Firefox is losing market share every year is being a great steward.