Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
Don't remove personal data if you uninstall the current version.
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.
Your bookmarks and other profile data are stored elsewhere (not in the Firefox program 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.
You may also have to remove the files in the updates and updates\0 folder:
C:\Documents and Settings\<user>\Local Settings\Application Data\Mozilla\Firefox\Mozilla Firefox\updates
(%USERPROFILE%\Local Settings\Application Data\Mozilla\Firefox\Mozilla Firefox\updates)
Re: Can not update or uninstall firefox because wizard says firefox must be closed to proceed with the update, as far as I can see fire fox is shut down.
To recap: tried unsuccessfully to update firefox so downloaded latest version, install wizard got as far as saying that firefox "must be closed to proceed" task manager did not show firefox listed under applications or processes, tried to uninstall firefox using windows add/remove programs same message "firefox must be closed to proceed with the uninstall" tried restarting computer, made no difference.
Fired up computer today and tried reload again - went ahead with no problem???? and I am now happily on the latest version. Many thanks to those who offered help, happy browsing.