Hello,

Did you disable those add-ons?

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 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 and select Options (Windows) or Preferences (Mac, Linux).
• Select the General panel.
• Under Performance, uncheck Use recommended performance settings. Additional settings will be displayed.
• 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.

I tried what you suggested but I still get the "this media could not be played" error

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

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

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

If you have Windows 10 N or KN then you may not have the Media Feature Pack installed.
https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows