For the last couple of days I was having a weird issue with Firefox on Ubuntu, namely there was no sound.
After searching all over the Internet and trying different solutions I finally found one that worked and I wanted to share it with you in case you find yourself in the same situation.
Note: before running the following command, please make sure you have a backup of your data!
Run this command in a terminal:
rm ~/.config/pulse/*
And restart your computer.
You should now have a working sound in Firefox.
This solution worked for me, Thank You very much. 🙂