First, read the Mozilla/Firefox recommendations on the download/install for their new versions, v1.0.3 could be installed over the top of v1.0.2 but it is recommended to do a clean install of v1.0.4.
My method for any download is to save to a folder named "Downloads" and within it I have sub folders with the individual named programs. I download the new Firefox version to a newly created sub folder then remove the previous version with Add/Remove then execute the new version. This way I always know where the execute is and I have a backup in case anything goes wrong. I can then delete the old version folder or keep it for reverting back to the previous version. This method of saving download executes works well for anything.
The new versions fixes some recently found vulnerabilities.:
My preferred method for installation is to uninstall the previous version from the Control Panel/Add-Remove Programs, then install the new version by double clicking on the downloaded installer file.
Hope this helps.