Mozilla is a strange beast. On the one hand, it's a non-profit organization who's mission is to make the web better and more open. Everything it does is geared towards this.
On the other hand, Mozilla is a company that tries to make money from its product, namely Firefox. And Mozilla Corp. the commercial arm, does make
quite a bit of money from the browser.
And some of that money goes towards advertising Firefox. In fact, the group ran a full-page ad in the New York Times with Firefox first launched.
So there's certainly a history there. But now, it started advertising the experimental versions of the browser as well, you may have noticed Firefox Aurora banner ads on some sites.
The idea seems to be to educate more users on the existence on these early Firefox builds, to get more to join and test them.