Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

There is no "plugin" to view HTML videos. That's the whole point of HTML5 - video playback by the web browser itself, without the need for any plugins. Or in the case of Flash, you don't need to install a separate plugin for Gecko / Plugin based or Trident / ActiveX based browsers.

With the recent changes made to YouTube pages all those YouTube add-ons you have installed may need to be updated.

I run no YouTube add-ons at all, and other that the Comments section loading in a delayed manner now vs a couple of weeks ago I am having no problems viewing the HTML5 videos.

And I haven't used the Flash 'toggle' in YouTube preferences for a couple of years now - HTML5 has worked fine since like Firefox 38 or even a bit earlier than that.

Thank you, I disabled everything and it works

I have the same problem and safe mode does allow playing html5. So it is an extension or plugin which causes the problem. BUT: trial and error is not a sophisticated way of troubleshooting.

I have a wealth of addons which make firefox usable to me compared to the ergonomically crippled default setup. Aren't there any log files which show me which extension may be the cause of the problem?

Is this problem on YouTube? Are you using any YouTube add-ons? Disable them.

Yes indeed, thanks for the hint. I had the youtube flash player extension and after I disabled it HTML5 playback works.

Glad to help. Safe Surfing.

Scaffery said

Thank you, I disabled everything and it works

That worked for me too.

I was having this same issue, then discovered I had the YouTube Flash Player add-on enabled in Firefox Preferenes. I disabled that and voila, videos now play within Firefox again.

How to get HTML5 video to play in YOUTUBE if it is "oddly incompatible with your browser". ADD to your ADBLOCKER. (copy & paste). |https://www.youtube.com/yts/jsbin/*/spf.js |https://www.youtube.com/yts/jsbin/*/www-pagead-id.js

I got a HTML5 video incompatibility fake error from YOUTUBE and am now watching HTML5 videos in the same Firefox 52.6 (garbage quality) browser after ADBLOCKING these parts of Youtube's defective webpages. It isn't your browser that is broken (except by stupidity on the programmer's part), it is YOUTUBE using this technique to censor videos.