Video format or MIME type is not supported

Question

This is the relevant code to run video:

Answer 1

FIXED IT!

I was losing my mind over this one. Reset firefox, tried safe mode, removed plugins, debugged using developers tools. All were to no avail and didn't get me any further with getting my online videos back to normal viewing condition. This however did the trick perfectly.

Within Firefox or whatever flavor of Firefox you have(CyberFox being my favorite choice here), simply browse to https://get.adobe.com/flashplayer/

VERIFY FIRST that the website detected you're using FireFox and has set your download for the flash player to be for Firefox.

Don't just click download. PLEASE PLEASE PLEASE SAVE YOURSELF the migraine and ALWAYS make sure that the middle section labeled "Optional offer:" is absolutely NOT CHECKED, it will be checked by default so always UNCHECK it before proceeding to download.

After it's finished downloading, close out of Firefox. Run the downloaded setup file As Administrator. It takes only a few seconds or so to complete, so after it's done, open up Firefox again and try viewing anything that was previously throwing this error. Should be back to normal now.

Enjoy!