When I try to install vFirefox v3.6.8, The update could not be installed. "msg says" Please make sure there are no other copies of Firefox running on your computer, and then restart Firefox to try again

I have Firefox ver 3.6.7 installed and when It try's to update to 3.6.8 - Install fails and with this phrase in a answer box " The update could not be installed. Please make sure there are no other copies of Firefox running on your computer, and then restart Firefox to try again." I have no other versions of Firefox loaded and I am not sure what to do? It seems to stay in this loop loading and then fails. Note: the green progress bar never seems to make it to the end. I end up having to cancel the install then Firefox 3.6.7 runs fine. This ver 3.6.8 is running with no issues on my laptop.