Without this fix, Firefox was essentially spending much of its time waiting to be allowed to draw, instead of being able to do other processing.
You can find more details on ROC's platform tilt post.
Who's to blame in this? Is it a case of malice or incompetence? David Hyatt (now with Apple, formerly with Mozilla) explains that Apple "had to do it to avoid performance regressions in apps that embedded WebKit". It may well be a gray area where the OS is not perfect and documentation is not up to date. Anyway, it has a bad taste of similar ugly practices back in the 90's that we have seen before.
There a joke saying that Apple and Microsoft are actually almost identical, except that one has a monopoly and the other has taste. Let's hope that Apple will not take a page of Microsoft's book about hidden APIs, so that the joke remains a joke...