Link:
http://blog.lizardwrangler.com/2008/11/19/sustainability-in-uncertain-times/

"....
In 2005 the Mozilla Foundation established a “tax reserve fund” for a portion of the revenue the Foundation received that year from Google. We did this in case the IRS (the “Internal Revenue Service,” the US national tax agency) decided to review the tax status of these funds.

This turns out to have been beneficial, as the IRS has decided to review this issue and the Mozilla Foundation. We are early in the process and do not yet have a good feel for how long this will take or the overall scope of what will be involved.
...."