ps -A | grep firefox | kill `awk '{print $1}'`
Warning: /boot/System.map-2.6.35.11-83.fc14.x86_64 has an incorrect kernel version.
The command have succeeded but with the warning.
The command before the previous one had not been successful. But there was also this warning.
You sure haven't given us much info here. But in general, ps looks at the kernel on disk to find out where everything resides. Then it looks at memory to see what is currently running. If you have updated the kernel on disk but you have not rebooted to get the new kernel in core, ps may notice that things don't make sense and issue a message like that. Another possibility is that the kernel in-core was trashed somehow.