Non-profits of the scale that Mozilla is need good talent to continue to exist. Good talent needs to be paid close to market rates to work for non-profits, and retaining good talent requires even better pay and benefits than just what will get good talent in the door

No matter how much or how little the talent at a nonprofit is paid people will go "why are they paying the CEO a $1 million dollar salary? They could hire 6-8 developers for that much!" "Why are they paying developers 100k/year? Can't they accept 80k for the privilege of working for such an important bastion of the open internet?"

15 million a year is a lot but it's also 1/3 the median CEO pay rate. They have to pay the CEO at least semi-competitively to retain them