Why is there an echo when multimedia files (mp4) inserted in my online help are played in FireFox? Should I be using a different video file format?

It may be the audio encoding of the file. (See Media formats supported by the HTML audio and video elements - HTML | MDN for an overview of supported codecs per browser.) MP4 should generally be safe. Since the video plays fine in IE and Chrome, I suspect this is a browser issue. I would advise you to also take this up with the producer of the video. Perhaps they have some futher insights.

Kind regards,

Willam