Internet Explorer and Firefox have special high contrast modes automatically activated when using a high contrast system theme. In this mode, background-image: is disabled which kills CSS sprites. This also removes the play arrow overlay hint on thumbnails. Recommend using font icons or embedded SVGs.
Kaltura Player: Normal and High Contrast mode
VideoJS player (dev): Normal and High Contrast mode
How to test in Firefox:
In about:preferences, scroll to Fonts & Colors in Language and Appearance, click [Colors...], under "Override the colors specified by the page with your selections above" switch from [Only with High Contrast themes] to [Always].