A couple of ways depending on the situation from a terminal sudo top
then kill the process you want. Another method is to alt-F2 and put
xkill into the run box on click on the app. With the firefox hassle,
open a terminal cd to .mozilla/firefox then ls to find the name of your
actual profile folder, it will have numbers and letters, cd to it, quick
way type in the first few and hit tab, this will auto complete. Once in
the folder ls -a and look for lock and .parentlock, rm both of them then
close the terminal and try again.