Great writing on this well-researched story, by Jason Hoffman:
At first glance, the bug appeared to be fairly routine, most likely a small problem somewhere in the website’s code or a strange coincidence. After just a few hours though, it became clear that the stakes for this one particular bug were far graver than anyone could have anticipated. If Firefox were to release this version of their browser as-is, they risked breaking an unknown, but still predictably rather large number of websites, all at once. Why that is has everything to do with the way MooTools was built, where it drew influence from, and the moment in time it was released. So to really understand the problem, we’ll have to go all the way back to the beginning.