To find out which machine Firefox is running on, type:
ls -l ~/.mozilla/firefox/*/lock
If there is a lock, it will be a symbolic link pointing to a file with the IP address of the computer where it is running followed by the process ID it is using. To translate IP-ADDRESS to a host name, type:
The process id should also be present, so type:
rsh <HOSTNAME> kill PID
~>ls -l ~/.mozilla/firefox/*/lock
lrwxrwxrwx 1 rivin dept 20 Jan 18 14:04 /u/rivin/.mozilla/firefox/uiil642a.default/lock -> 126.96.36.199:+24876
188.8.131.52.in-addr.arpa domain name pointer eagle.cs.utexas.edu.
~>rsh eagle kill 24876
Be sure to remove the lockfile if killing the process didn't!
If this doesn't work, you will need to delete the .parentlock file: