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

If this doesn't work, you will need to delete the .parentlock file:

~>rm ~/.mozilla/firefox/*/.parentlock