You can't, AFAIK. Firefox is run by the non-profit 501(c)3 Mozilla foundation. You can make a nice tax-deductible donation though, which will improve features, code quality and support.
The unavailability of contracted support is one of the reasons we haven't switched to Firefox or any of the other similar browsers. We turn a blind eye to folks who want to use it, but we only provide support for IE (much to my chagrin). There are possibilities in the future that I can't discuss, but corporate America just can't depend on open source without contracted support that we can measure and count on. And we're not staffing up to be able to support it ourselves. Quite the opposite, in fact.