Open a terminal window, maximise it and run
Your terminal window should fill with the top (a standard UNIX) application.
to get help with configuring top - viz. using 's' to set the update interval from its default 3sec to 1sec. Then open Firefox and watch how top changes to see if there is a marked increase in CPU or RAM usage. If that doesn't give you clues about what caused Firefox to crash, you could always try looking at the log files in /var/log.