See:

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

Remove the files in the updates and updates\0 folder.
You may need to delete active-update.xml and updates.xml as well if present.

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

If you have problems with updating then easiest is to download the full version and uninstall the currently installed version.
You may need to remove the Firefox program folder to do a clean install of the new version.

• Download a fresh Firefox copy and save the file to the desktop.
• Firefox 4.0.x: http://www.mozilla.com/en-US/firefox/all.html

Thanks but removing the files didn't help at all. Wayne

Did you ever install a Firefox 4 beta or release candidate?

If you have problems with updating then easiest is to download the full version and uninstall the currently installed version.
You may need to remove the Firefox program folder to do a clean install of the new version.

Do a clean (re-)install:

• Download a fresh Firefox copy and save the file to the desktop.
• Firefox 4.0.x: http://www.mozilla.com/en-US/firefox/all.html

• Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
• Do not 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 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.