Display bug with the video player in CoursePress on Firefox

Hi there,

I don't know if it's related to CoursePress Pro, but it's happening there. There is a display bug on the video player when using Firefox :

It seems that the "white arrow" sign that is supposed to be at the center of a video is much down, which is weird. See picture attached to understand the problem better, and hopefully tell me how to fix it.

Please let me know, thanks a lot.