uninstall previous version first then install new one. All your bookmarks,settings,etc will still be in effect as they are saved in a separate folder from the main program. If you search the Firefox site you will find this is the recommended way.
re. extensions/themes. You will probably have to reinstall these as they do not transfer to new installations. Or possibly even work. This has been the case with previous upgrades but who knows this time it may be different.