maybe cost of $10000s (or even $100K+) to build. this is where your donations go towards!
Honestly, short of going and looking it up, I haven't got a clue what the for-profit mozilla entity does other than the browser. Sign deals exchanging money for the default search engine position?
Why would that stand to reason? Firefox makes Mozilla money (through search provider sponsorship), and therefore needs to be a company to receive that revenue. Nothing else Mozilla does makes them money, and therefore nothing else they do needs to be a company. What would you imagine the Mozilla Corporation exists to do, if not to make money from Firefox (and then plow that money back into Firefox development)?