According to the comments, the IRS grants automatic extensions to October 15th (for non-profits). It is (again, according to some commenter) normal for a non-profit to issue financial statements this late in the following year.
Do keep in mind I was unable to vet this reason within a couple minutes of searching.
I'd say the 88% of profits coming from Google is in line with how much Google owns of the search market. It's not a particularly stunning number; if the search market was more distributed, Mozilla's revenue would be more distributed.
This is why you have this thing called a "fine." IRS complains, says Mozilla Foundation has to pay a fine. Google gives Mozilla money, IRS gets paid, everything is "fine."
Maybe their finance software is written in XUL.