Does it still happen in Troubleshoot Mode?

No it did not.

I have refreshed it, didn't work.

I have re-installed it, didn't work.

I now have to use Edge as my streaming browser, and I'm not very happy about it.

You can try to disable hardware acceleration in Firefox.

• Settings -> General -> Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting to apply changes.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

This problem started when I upgraded my monitor, the old one was connected via a VGA cable / HDMI adapter and the sound was direct from the PC. The sound is still the same but teh monitor is now directly connected with a HDMI cable. I have tried using the sound from the monitor earphone socket to my small sound bar but nothing is coming out of that. Firefox is the only browser affected with this problem. Edge works fine when streaming TV and movies.

Visit about:support in the address bar and check the "Output Devices" section.