Firefox: Dragging slider in replay does not update markdown until released

[markgeraty]

Dragging the slider in chrome continuously updates the markdown, but in firefox it does not update the markdown until the slider is released.

Issue seen in Firefox 37 (working in Chrome 41).

[markgeraty]

May have something to do with this issue.

[ctsmith95]

It seems to update in real-time when the slider is moved slowly. It might just be firefox struggling to keep up with applying the changes to the codemirror instance.