The callback logic record/replay logic is used to record/replay library calls which can reenter Gecko via callbacks passed into them. This has apparently been dead code for a while (which will happen if the library stops being called by content processes) as it has some bugs which will always lead to a crash, and for some reason we have started hitting those crashes on TodoMVC.
The associated patch fixes the callback bugs so that TodoMVC records/replays correctly. There is one potential lingering issue, though. We will crash if these callbacks are invoked after diverging from the recording, and in the case of CGPathApply this can definitely happen while repainting. I haven't found a way to get such a crash to occur yet, and it seems better to fix in followup as dealing with it properly is more involved.