If firefox fails to shut down properly, it may leave behind a lockfile that will prevent you from opening a new browser session.

To fix this we must remove the file .parentlock from your firefox profile. This is located in your home directory ~/.mozilla/firefox/{random}.profile/.parentlock. Your profile path will contain a random string, so we must locate it

ktm5j@portal02 ~ $ cd .mozilla/firefox/
ktm5j@portal02 ~/.mozilla/firefox $ ls -l
total 5
drwx------ 11 ktm5j compstaff  53 Mar 20 12:23 bg4x2fi2.default                  <- find this folder
-rw-r--r--  1 ktm5j compstaff 104 Feb 11 12:46 profiles.ini
ktm5j@portal02 ~/.mozilla/firefox $ cd bg4x2fi2.default/                         <- cd into it
ktm5j@portal02 ~/.mozilla/firefox/bg4x2fi2.default $ rm .parentlock 

You should now be able to start a new firefox session.