Make sure you are not blocking content.

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?

FredMcD said

Make sure you are not blocking content. 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?

Thanks for replying, but I tried this, works fine on the first video, then it happens again.

The first clip plays, but not any others?

Please provide public link(s) (no password) that we can check out. No Personal Information Please !

FredMcD said

The first clip plays, but not any others? Please provide public link(s) (no password) that we can check out. No Personal Information Please !

I re-check and the conclusion is: - safe mode: everything works fine

Then I start in normal mode: no videos at all. The same with incognito mode.

This video worked in safe mode and did not work in regular mode: https://www.youtube.com/watch?v=gokLI42w_WI

I have the following add-ons:

OpenH264 Video Codec provided by Cisco Systems, Inc Widevine Content Decryption Module provided by Google Inc. hockwave Flash

Is there any other thing worth checking?

Thanks

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). If the problem goes away, that tells us one thing. If it does not, that tells us something else.

It could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

You can check the Web Console (Tools -> Web Developer) for media related messages and messages about the presence of mixed or unsafe content or Tracking Protection.

• https://developer.mozilla.org/en/Tools/Web_Console

FredMcD said

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). If the problem goes away, that tells us one thing. If it does not, that tells us something else.

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It could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Thank you! Disabling Hardware Acceleration seemed to solve the problem.

Glad to help. Safe Surfing.