lucy24 - 8:40 pm on May 15, 2012 (gmt 0)
Ooh, it's the "thumb.php" robot. I've met them. Technically you don't need to block them at all, since they're already getting a 404. But a 403 is more satisfying.
In most circumstances, Firefox 3.anything is a robot. But be careful, because there are exceptions. (I'm one of them myself.) Firefox 3 + Windows is probably a safe lockout.