where <username> is replaced by your MathLAN? username
and <profile> is replaced the name of your profile directory
The profile directory's name should be a random string of characters appended by the name of the firefox profile that you are using, e.g. "87a41wh6.default".
So the full pathname for your profile directory will be something like /home/frenooli/.mozilla/firefox/87a41wh6.default (if frenooli were your username).
Within that directory, remove the files: lock and .parentlock
rm .parentlock lock
Congratulations! You've removed your firefox profile's lock file!