<disclaimer>I'm a Moco employee</disclaimer>
I don't know who told you that Mozilla was required to spend all $300M each year, but looking at the financial from past years will show you that it's not that clear (https://www.mozilla.org/en-US/foundation/annualreport/2013/)
The apartment in Paris is actually a way to spend less compared to sending people that visit the Paris office to hotels.
Bonuses are not >40%/yr, this is the absolute maximum, and it has never been 40% per year in the last 4 years. It's true that compensation at Mozilla is good, but you need that to be competitive on the job market.
Donations are important for the Mozilla Foundation to keep a non-profit status. The Yahoo! deal is a Mozilla Corporation deal, so that's a different story (Mo Corp pay taxes like any company).
Yep, they have already went through one IRS audit because too much of their income was concentrated on too few sources (makes it look like a front set up by those sources for tax purposes). See https://en.wikipedia.org/wiki/Mozilla_Corporation#IRS_audit for example.
Donations are important for the Mozilla Foundation to keep a non-profit status. The Yahoo! deal is a Mozilla Corporation deal, so that's a different story (Mo Corp pay taxes like any company).