How is HTML5 vs Flash determined when a user plays a course?

So I'm trying to understand why and when these modules decide to use either HTML5 or Flash. It's my understanding that Storyline will default to Flash and then fall back on HTML5 when the browser doesn't support whatever is going on.

An issue I'm facing is that some of my client's users are experiencing issues with slider interactions in Firefox. The slider will either freeze, not go backward, or fly to the opposite side of the bar and not move. From my googling, the answer seems to be because Storyline does not support HTML5 in Firefox, which is fine. But why isn't Firefox using Flash? Is the slider interaction only available through HTML5?

I feel like once I get to the bottom of this, I'll be able to fix it. Simply switching browsers is not entirely an option at this point. (Ultimately, it may have to be, but we'd like to try avoiding that for the time being).

Thanks for your help!