This indicates an attack attempt against a command execution vulnerability in DXStudio Firefox Plugin.
The vulnerability is caused by an error when the vulnerable software handles a malicious shell.execute script. It allows a remote attacker to execute arbitrary command via sending a crafted web page.
Worldweaver DX Studio Player 18.104.22.168, 22.214.171.124, 126.96.36.199, and probably other versions before 188.8.131.52
Upgrade to DX Studio Player version v184.108.40.206 or later.