Mozilla is a non-profit organization. Any money that they make, they have to reinvest into Firefox or other Mozilla projects. There's not some guy getting rich from you helping them out. All the help that you invest will eventually fall through to users in some way.
Mozilla Foundation (MoFo) is non-profit. Most of the employees work for Mozilla Corporation, which is owned 100% by MoFo.
It's very clear the MoCo doesn't reinvest all the money they make (they have a lot in the bank). And some are getting rich from it, but that's a different topic.
I don't consider getting paid your wage as "getting rich", if that wage is within industry standards for the job that you're doing.
And yes, the MoCo is not reinvesting all money all the time right away, but they'll have to reinvest it or use it to pay wages at some point. They have no reason to just amass money senselessly.
If they make a surplus, have all wages paid, and don't save it up nor reinvest it, then they can only pay it out to the Mozilla Foundation, where it's again in nonprofit hands.
To be completely fair: many projects at many companies turn out to be failures. Rust could've been bad and Firefox OS could have ended up ruling the world, and suddenly your comment would look very different. There's just a fundamental risk in projects, and viability often only becomes later in hindsight. (Most other companies failures just happen to be a lot less visible since they're not developing in the open.)
