See http://kb.mozillazine.org/Software_Update (Software Update not working properly) 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)
C:\Program Files\Mozilla Firefox\updates

I deleted the files and directories as the information at http://kb.mozillazine.org/Software_Update instructed and the message still appears. There is obviously more to this than simply removing those two files and the directories that hold them.

Did you also delete the Firefox program folder to do a clean reinstall?

It sounds that some files are still in use and can't be replaced by the updater.

You may need to delete the updates folder as well to remove files that were downloaded.

That is what I was first inclined to do, but I have several addons installed and do not have a list of them written down nor do I have them memorized and I do not want to have to figure out what they all were all over again. So I came here to ask the question hoping to get an answer that would allow me to get past this problem without resorting to the scorched earth approach.

Just to see if it would work and not remove all of the addons that I have installed I renamed the Firefox program folder C:\Program Files\Mozilla Firefox to Mozilla Firefox-old and ran the Version 3.6.10 installation program. It ran without a hitch. The addons that I had installed are still present so I am guessing that they are not stored in the installation directory. I am also guessing that something in that directory tree is corrupted. I deleted it and was able to start a new instance of Firefox. So it looks like this is solved.

I need to make this happen for me in Windows 7. Can you help?

See http://kb.mozillazine.org/Software_Update (Software Update not working properly) Remove the files in the updates and updates\0 folder:

For Vista/Windows7 you can look here: C:\Users\<user>\AppData\Local\Mozilla\Firefox\Mozilla Firefox\updates (%LOCALAPPDATA%\Mozilla\Firefox\Mozilla Firefox\updates)

C:\Users\<user>\AppData\Local\VirtualStore\Program Files\Mozilla Firefox\updates

AppData is a hidden folder.
See http://kb.mozillazine.org/Show_hidden_files_and_folders

C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\xuab2q88.default

This is the pathway I find after unhiding all files and folders.

That is the path of the Firefox Profile Folder.

Did you look if you have this folder?

C:\Users\<user>\AppData\Local\Mozilla\Firefox\Mozilla Firefox\updates 

If Firefox updates then the files will be stored in that location.

I'm telling you, there IS no "\Mozilla Firefox\updates" folder. None. All my files and folders are unhidden. I cannot do a reinstall over the existing attempt because it routes right into "requires a restart". I am absolutely lost here.

C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\xuab2q88.default is the location of the cache folder for your default profile. If the updates folder isn't there then it looks that Firefox hasn't downloaded an update.

You can download the full version from http://www.mozilla.com/firefox/all.html

Delete the Firefox program folder: C:\Program Files\Mozilla Firefox That will remove all Firefox program files in case some are locked or protected by security software.
That should allow you to install a full version of Firefox 3.6.10

Your Firefox profile data is stored elsewhere and won't be affected.

Sold. Interesting that I could delete through Program Files but that Windows wouldn't find it through "Add/Delete".

All is well and the upgrade is installed with all addons working.

Thank you for your assistance. It'll teach me to upgrade more than one thing at a time. I got the "Critical Windows" fix too.

You're welcome.

Renaming the Mozilla Firefox directory, reinstalling, and deleting the old directory seems to have worked for me too.

The only difference is hat the default install directory was "Program Files (x86)" vs "Program Files".

Thanks for the tip.

So, for better understanding... if i delete the folder Mozilla Firefox from C:\Program Files\Mozilla Firefox and install a new full version, i will get a working firefox with my old bookmarks ?? or shall i backup them first ?

You won't lose bookmarks and other profile data if you delete the Firefox program folder, because that data is stored elsewhere in the Firefox Profile Folder.

I can't guarantee that deleting that folder will make Firefox work, but chances should be good if it did happen because some files couldn't be updated.

This worked, THANK YOU!