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:
host <IP-ADDRESS>
The process id should also be present, so type:
rsh <HOSTNAME> kill PID
For example:
~>ls -l ~/.mozilla/firefox/*/lock
lrwxrwxrwx 1 rivin dept 20 Jan 18 14:04 /u/rivin/.mozilla/firefox/uiil642a.default/lock -> 128.83.120.48:+24876
~>host 128.83.120.48
48.120.83.128.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!
~>rm ~/.mozilla/firefox/*/lock
If this doesn't work, you will need to delete the .parentlock file:
~>rm ~/.mozilla/firefox/*/.parentlock