There's no indication of its use on a global scale just yet, however due to the fact that the PoC is now public, it shouldn't take long before cybercriminals embed it within the diverse exploits set of their web malware exploitation kits, allowing it to scale.
"Mozilla Firefox is prone to a remote code-execution vulnerability. Successful exploits may allow an attacker to execute arbitrary code in the context of the user running the affected application. Failed attempts will likely result in denial-of-service conditions. The issue affects Firefox 3.5; other versions may also be vulnerable.
NOTE: Remote code execution was confirmed in Firefox 3.5 running on Microsoft Windows XP SP2. A crash was observed in Firefox 3.5 on Windows XP SP3."
Additional testing courtesy of heise Security indicates the exploit crashed Firefox under Vista, and that when tested under Windows 7 RC1 a dialog abortion script appeared.
In terms of mitigation, NoScript works like charm, successfully detecting the PoC's attempt to access file://.