The Mozilla Foundation is a non-profit. The Mozilla Corporation, which actually builds Firefox, isn't. So the tax status of the organization is a bit tricky. Obviously, Mozilla was trying to minimize the taxes it had to pay to the US government.
But the IRS wasn't so sure that Mozilla was truly tax exempt, and if it wasn't, it didn't know how much it had to pay.
Which is why it started an audit way back in 2008. That audit has finally been concluded and the IRS asked the Mozilla Foundation to pay $1.5 million, €1.17 million in taxes, after a settlement between the two sides.
Mozilla, fearing the worse, had put away $15 million, €11.7 million in case it was ordered to pay back taxes. Since that's not the case, that money can now be put to good use at the foundation.
It's a big sum, but considering that Mozilla is said to be getting some $1 billion, €780 million from Google over three years, it's safe to say Mozilla is not exactly hurting for cash.