They publish their audited financial statements online:

http://www.mozilla.org/foundation/documents/

http://static.mozilla.com/moco/en-US/pdf/Mozilla%20Foundatio...

As of December 31, 2010, they have assets of $168MM, of which $35MM is in cash and $105MM in investments. In the 12 months they spent $62MM on software development, $10MM on branding and marketing, and $12MM on 'General and administrative'.

There's more granular detail if you're interested.