Many sites have transitioned to serving HTML5 video in the "closed" MP4 format. For licensing-related reasons, Firefox does not yet have a built-in MPEG decoder. When Firefox runs natively on Windows 7, it uses Microsoft's Media Foundation libraries for MPEG decoding. If Open Kiosk does not have access to those libraries, that could explain why it is running into this issue. However, I can't find much documentation on it, and I haven't tried it myself.

Separate Issue; Your System Details shows;

Installed Plug-ins

Adobe PDF Plug-In For Firefox and Netscape 15.8.20082 Adobe PDF Plug-In For Firefox and Netscape "9.5.5"
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Security Issue: Update your Adobe Reader Version 2015.016.20039 Note: Other Software is offered in the download.