Engage interactions suddenly not playing in some browsers

I'm an instructional designer at Oregon State University, and we use Engage activities in several courses. Suddenly this term, a few of our activities are "hanging" rather than playing through, but only on certain browsers. We've used these activities successfully for two terms, and this error is just cropping up now in term three. 

Here's an example: http://oregonstate.edu/instruct/dce/waterwise/module1/cs2/player.html

This example plays in Chrome, but not in Firefox. Previously, it did play in Firefox. Any idea what's happening? 

Thanks,

Shannon