Hi steen.billenstein, please ignore that spam message with a phone number. That is not official and we have no idea what they will do to your system if you engage with them.

The video that doesn't play in mp4v: Codec: MPEG-4 Video (mp4v) The video that plays is avc1: Codec: H264 - MPEG-4 AVC (part 10) (avc1)

Maybe consider to re-encode this video in a supported format.

There is a bug report about supporting mp4v:

• Bug 1414402 - Support playback of mp4v video

Please do not comment in bug reports

• https://bugzilla.mozilla.org/page.cgi?id=etiquette.html

You can vote for a bug to show your interest

• https://bugzilla.mozilla.org/page.cgi?id=voting.html

Thanks to all Helping People.

What I did, was to re-encode my video, and uploaded the new video to my own homepage. And now it's work.

In fact, I don't know the difference of the two codec. The first one has been made in 2015 with Pinnacle Studio 12 and the new one made to day Pinnacle Studio 19 ...

The surpricing question is: Why did the old one show ok until some weeks/month ago ??

One issue that seems more common lately is Firefox's Tracking Protection feature. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature: Tracking Protection {web link}

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

You will need to restart Firefox for this to take effect so save all work first (e.g., mail you are composing, online documents you're editing, etc.,) and then perform these steps:

In Firefox 54 and below:

   Click the menu button New Fx Menu and select Options (Windows) or Preferences (Mac, Linux).
   Select the Advanced panel and the General tab.
   Uncheck Use hardware acceleration when available.
   Close Firefox completely and then restart Firefox to see if the problem persists. 

In Firefox 55 and above:

   Click the menu button New Fx Menu and select Options (Windows) or Preferences (Mac, Linux).
   Select the General panel.
   Under Performance, uncheck Use recommended performance settings. Additional settings will be displayed.

   Fx55Performance-disableHWA

   Uncheck Use hardware acceleration when available.
   Close Firefox completely and then restart Firefox to see if the problem persists. 

Did this fix your problems? Please report back to us!

If the problem is resolved, you should check for updates for your graphics driver by following the steps mentioned in these Knowledge base articles:

   Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems 

   Upgrade your graphics drivers to use hardware acceleration and WebGL 

Thank you.