Bug 1207355 introduced some additional fuzz on OS X 10.10 only on the tests in layout/reftests/generated-content/reftest.list. I suspect, but have not yet proven, that this is the same issue as bug 1128229. Once optimizing imgFrame's happens synchronously, we should remove this fuzz. If the problem persists, that will indicate that something else is happening, but I doubt it given the symptoms.
So a try push that never optimize frames would not show the problem then? Have we verified that?