How can I solve the "browser does not currently support any of the video formats" message?

This problem occurred when I watch a YouTube live video with Firefox on Ubuntu.

Installing ffmpeg solved my problem:

sudo apt install ffmpeg or

Same fix on Fedora 26 Workstation with Firefox 56 (x64):

sudo dnf install ffmpeg

Tyler, as others have mentioned Firefox supports HTML5 video natively. If your Firefox installation does not, please try the following steps:

First, disable all the add-ons in your browser and try visiting Youtube again. If Youtube works, you can go through and enable single add-ons at a time, testing each time to make sure video plays correctly.

Next, if you're still not able to play Youtube videos natively, remove Firefox entirely and then go to https://www.mozilla.org/en-US/firefox/new/ and download a fresh copy of the latest version and install it.

UPDATE: More clarity on HTML5

HTML 5 is essentially a container, a WAY to deliver video, it is not the video codec itself.

In the picture you show, the HTMLVideoElement is the code that Youtube uses that the browser knows how to handle, but it doesn't actually encode the video, it's just the container the video is delivered in.

The video itself must be encoded in one of several formats, which are the other parts of the picture: h264, WebM, MSE, etc.

So, your browser knows how to handle the container, but it should also know how to handle the encoding formats itself, and that is where your problem lies.

If you look into the Stats For Nerds that @Scott Chamberlain mentions in his comment above, you will never see HTMLVideoElement listed as the encoding format. You will, however, find each Youtube video is encoded using h264 or WebM or MSE, and, once again, this is what your browser is failing to do and what you must troubleshoot, and the steps I've outlined above are the correct first steps to doing so.

My FIX: I went to https://www.youtube.com/html5 (Thx Grandpa Dave!), - Scrolled to the BOTTOM YOUTUBE LOGO on left- and just to the RIGHT of the bottom-left logo "LANGUAGE" and CORRECTED my location/LANGUAGE.

Everything played fine after that. Pls share if this worked for you. It's a weird auto-location selection. I thought there might be a prob when the comp auto-filled my date/time with another country time, sure enough, youtube matched the incorrect auto location.