Well, Firefox has reached fifty million downloads. Although some of these downloads probably take into account the recent patches that have been rolling out. However, I think these might be cancelled out (OK, not quite, but a dent into them) by people installing versions over networks to give to their friends and families. I know that's the situation here at least.
For commemoration spreadfirefox.com are releasing fifty special edition coins to people with the best/most amusing/far out story of how they went about "spreading firefox".
They've also got a "something special" for the affiliate who delivered the fifty millionth click. Although, they haven't divulged what it is, yet.