I suggest the following...

WebM videos not working in latest Firefox

Videos are not displaying in the latest Firefox version. Perhaps this will help:
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/video#Server_support

1 vote
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    Fred shared this idea  ·   ·  Admin →

    2 comments

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      • Fred commented  · 

        "No video with supported format and MIME type found." is the error message I get on Hiddenlol.

      • Fred commented  · 

        They usually result in 404s, while on Chrome they work.

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