If you click through to the bugzilla item Block cookies from sites I haven’t visited, there’s a debate going on about it. The change essentially makes FF behave like Safari. There is some worry that FF, representing roughly 10x the traffic compared to Safari, could upset many people with this change. Would that result in a drop in Firefox usage? Maybe there non-advertising usages for 3rd party cookies, but it seems unlikely that the would be to domains you’ve never visited. Chrome, naturally, will remain uber-friendly to anything advertising related, so this might be a differentiator for Firefox.
On an unrelated note, Mozilla still uses Bugzilla and Mailman.