i had the same problem, our network has more than 20 users, and it keep doing this every few days for one of the users, and its a headache for me to remove the lock files manually everytime,
this small shell script will do the work for you, i placed it in ~/.kde/Autostart/ so in case this happen, the user has just to log off and on again to get firefox and thunderbird running again.
#assuming profile name is made by firefox and ends with .default
#you can get correct profile name from profiles.ini
rm -rf ~/.mozilla/firefox/*.default/lock
rm -rf ~/.mozilla/firefox/*.default/.parentlock
rm -rf ~/.mozilla-thunderbird/*.default/lock
rm -rf ~/.mozilla-thunderbird/*.default/.parentlock