Maybe you can email them and ask?

Marketing is not just ads. They support a lot of projects like the Stupid Shit No One Needs & Terrible Ideas Hackathon[1], the Gigabit Community Fund[2], the Hive NYC Learning Network[3], and many more, which probably are itemized as marketing.

[1] http://stupidhackathon.github.io/

[2] https://www.mozilla.org/en-US/gigabit/

[3] http://hivenyc.org/

The discussion of those numbers is here:

https://static.mozilla.com/moco/en-US/pdf/Mozilla_Audited_Fi...

It isn't hugely informative, but I would guess that the $9 million in office leases mentioned towards the end is categorized as an administrative cost.