Hi,
are you using the latest krpano version 1.17?
The latest Firefox browser and the latest Windows Chrome browser are reporting to the videoplayer plugin that they are supporting MP4 but in real the don't do.
For this reason the videoplayer.js plugin from the latest krpano version will prefer 'webm' by default (when 'webm' support was reported from the browser). The webm format is typically working fine in both browsers.
The online example here should work in all browsers:
http://krpano.com/plugins/videoplayer/#examples
Best regards,
Klaus