how you've described it, you have just downloaded version 12 of firefox but not installed it yet. go to tools > downloads (or press ctrl + j) & double-click the firefox 12 setup file in order to run the installation (you might have to confirm a warning message that you're about to run a .exe file after that).

then follow the steps as they are described in How to install Firefox on Windows

edit: afterwards you should also update your flash plugin from https://www.adobe.com/go/getflashplayer (uncheck the option to install the bundled security software)

If files already have been downloaded then remove the files in the updates and updates\0 folder.
You can also delete active-update.xml and possibly updates.xml (stores the update history).

• http://kb.mozillazine.org/Software_Update (Software Update not working properly)

C:\Users\<user>\AppData\Local\Mozilla\Firefox\Mozilla Firefox\updates
(%LOCALAPPDATA%\Mozilla\Firefox\Mozilla Firefox)

If there are problems with updating then best is to download the full version and uninstall the currently installed version and delete the Firefox program folder to remove any leftover files.

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 12.0.x: http://www.mozilla.org/en-US/firefox/all.html

Uninstall your current Firefox version, if possible.

• Do NOT remove personal data when you uninstall your current Firefox version, because all profile folders will be removed and you will also lose your personal data like bookmarks and passwords from profiles of other Firefox versions.

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.
• http://kb.mozillazine.org/Uninstalling_Firefox

Your bookmarks and other profile data are stored elsewhere in the Firefox Profile Folder and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

Brilliant advice - everything seems fine now. Thanks