Out of the blue I'm getting and error of :"The update could not be installed. Please make sure there are no other copies of FF running on your computer, and then restart FF to try again." I tried to do a manual update thru Help-About FF and keep getting the same error. Was doing this a couple of months ago then quit and now it's back. I'm on a new laptop running Windows 7 and all my info was transferred from my old to new computer by a professional.
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 if there are still problems.
Uninstall your current Firefox version, if possible, to cleanup the Windows registry and settings in security software.
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.
You may need to 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.
Your bookmarks and other profile data are stored in the Firefox Profile Folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox.