firefox.exe hangs when I shut it down. How do I fix this?

I’m running V3.6.16 but this has been happening for several months. After shutting Firefox down, I find that other programs run very slowly. When I check Task Manager, I find that firefox.exe is using more than 90% of the CPU capacity, I have also found that when add-on updates require a restart of the program, it shuts down but rarely restarts. I have let the computer sit for as long as half an hour to see if it just needs more time to complete the process but this has made no difference. Any thoughts on how to correct this?