Het forum

Installed 4.16 via ubuntu software center, updated to 4.18 and still get firefox error for this.

This url:playonlinux://www.playonlinux.com/repository/download.php?id=1141

"Firefox doesn't know how to open this address, because the protocol (playonlinux) isn't associated with any program."

You need to have gconf installed, and the deb package should configurate the protocol