Because Firefox forces all of it's internal workings to run in a single thread the causes for this can be many but most of the time it is caused by a dodgy extension or plug-in, running any old or unusual plug-ins?
Anyway the only decent way to track down the culprit is to disable all the plug-ins you have and do a shut-down, start-up while watching the processes screen in system properties to see if it shuts down faster, if it does then you should re-activate your extensions/plug-ins one by one until you find the culprit.