What percentage of Mozilla revenues are spent on Firefox development?
50% directly spent on Firefox development.
12% spent on marketing (for comparison, rumor has it that Chrome's marketing spend is comparable to Mozilla's total revenue).
13% general and administrative expenses (this includes things like leases on office space, as far as I can tell from the usual definition of this category).
9% (of revenue; it's 36% of profit) income taxes. Which Mozilla apparently pays, unlike some other tech corporations.
16% money going into the rainy-day fund.
Disclaimer: I work for Mozilla; pardon my snark about taxes. It's a sore point.
The Mozilla Corporation is a wholly owned subsidiary of the Foundation. It is _not_ a nonprofit and develops a web browser as well as most other software you would associate with "Mozilla". It pays dividends to the Foundation which the Foundation uses, alongside donations, to fund its work. The Corporation pays taxes on its profits, like any other corporation that is not explicitly a nonprofit. Note that I am explicitly not using "for-profit" to describe the Corporation, because profit is not the Corporation's goal. Again, the only shareholder in the Corporation is the Foundation, though they do have separate boards and somewhat separate governance.
Google advertises Chrome on the Google start page, literally on google.com. I wonder how much that web real estate would go for if Google were to auction it off?