Reset the software update feature by deleting the files and folders as shown in this article: http://kb.mozillazine.org/Software_Update

thanks for the advise. I did just as you said (deleted the update directory and the 2 mentioned files) and the problem went away. Now the Updater says there is an update waiting. I cliked on it downloaded and then tried to apply the update. It said it failed and it would download the full version of the update which I saw was 10 meg or so. I downloaded that and applied it and now I am back to square one. I get the "Software Update failed, please be sure this is the only instance of Firefox running and try again". I dont have that choice because it goes back in the endless loop.

In my situation the solution was simple "I just hit CANCEL" on the update plate. No more problems. Looks like a bug. This bug is brought about because the update program itself is being perceived as and "open Firefox application" which in turn causes the error to occur. Solution is to make the install program "look like a non Firefox application", then the update will succeed.

I deleted the files and folder as instructed, but this problem comes back every time it needs a software up date. Does anyone have a permanent solution? I am running XP.

I deleted the files and folders and this seems to have helped the first time, but the problem is back bigger and badder than before. Does anyone have a permanent solution?

Do a clean (re)install of the latest Firefox version.

• Download a fresh Firefox copy from http://www.mozilla.com/firefox/all.html and save the file to the desktop.
• Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
• Don't remove personal data when uninstalling.
• 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.

Remove the files in the updates and updates\0 folder:

XP:
C:\Documents and Settings\<user>\Local Settings\Application Data\Mozilla\Firefox\Mozilla Firefox\updates
(%USERPROFILE%\Local Settings\Application Data\Mozilla\Firefox\Mozilla Firefox\updates)

Vista/Windows7:
C:\Users\<user>\AppData\Local\Mozilla\Firefox\Mozilla Firefox\updates
(%LOCALAPPDATA%\Mozilla\Firefox\Mozilla Firefox\updates)

Has this actually worked for anyone permanently?

Even late in 2011 programmers believe that everyone logs in as administrators.

If you log in every day as a Standard User (as you should) then I've found that the prompted Firefox Update simply will not work. If you are logged in as an administrator I've found that the update works every time - reasonably enough: Users can't write, install, to protected areas of the disk; administrators can.

Mozilla really should prevent the 'Update' button showing if it's not going to be possible to succeed as an ordinary user.

So try it. If you can log in as an admin, and you're not, swap, install. Then swap back as a User and continue practising safe hex. If you can't log in as an admin, then it's not your role to update the programs. Call in someone who can. A son or daughter, perhaps.. :)