The latest IRS Form 990 for the Mozilla Foundation I can find is here:

https://static.mozilla.com/moco/en-US/pdf/2013_Mozilla_Found...

It's two years old, but if you go through it you'll see some very large numbers in the officer compensation lines, but it's mostly from "related organizations" and not the foundation itself. I'm not really sure how to parse it, and it may have changed in the last 2 years.

Admittedly, its all off record hearsay as it's not to smart to talk badly about your C-levels in public discussion. Though I must say, as a user its hard to sincerely distinguish the business motives of Firefox from other parts of the org. Especially in the recent push of the browser.