Q: After upgrade to 20.04, no sound from Firefox I am getting sound from other applications I've tried (Chromium, VLC, Videos), but nothing from Firefox.
Everything looks like it should be working. In sound settings, Firefox is turned up to 100% and when I play something, the output device even lights up like it thinks that sound is coming out, but it isn't.

A: Install pavucontrol:
sudo apt install pavucontrol

Select required sound device on the Playback tab if a few are installed:

Try different output channels on Output Devices tab if a few are available:


A: In Gnome (Debian buster) I had the same problem on Youtube with Firefox only (system sound/Chrome and any other application was ok): I had to play a Youtube video, while playing the video I checked the Gnome sound system parameters and select "Applications" tab (you should see Firefox in the list), there the sound was only set to about 10%, I have no idea why...I suspect that this sound parameter has been changed by using VLC volume control...
