@dom333 I would recommend running through the standard troubleshooting steps for your Firefox browser to see if it helps with adding songs to your playlist. You can find those steps here: Firefox Troubleshooting
An alternative is to try using the desktop application on your computer to see if that works for you. You can find out more information about how it works here.
If you continue to have trouble with this, would you mind posting a screenshot in your next reply?
To take a screenshot, you can try using theSnipping Tool.
You can find specifics about this tool and how to use ithere.