I have had problems with FF in the past. It turns out my Profile folder got corrupted. This tells you were the profile folder can be located in various systems: click here and how to manage the folder.
I've found that you can install updated versions over your old ones but you end up with 2 folders installed.Best to uninstall the previous version first by way of add/remove programs in the control panel to prevent any conflicts. All your settings and bookmarks etc are retained in a separate folder when you do this.