Maybe a dry run ?
Conspiracy theories with China are certainly interesting, but I'm thinking of a different scenario.
By hacking a site that is high-profile with a 0-day, the miscreants had to know that their attack would be quickly discovered.
I fear that they chose the site on purpose to evaluate what the reaction time would be, and what the damage would be.
Given that they chose to attack Firefox, I deduce two things : first, we will actually have a fix in the next 24 hours, instead of 24 months for IE. Second, Firefox is now important enough to be attacked on its own in a high-profile attack, instead of IE.
So Firefox and NoScript are the two things that really make the web safer.