Could you try to install Firefox by downloading the full installer from www.mozilla.org instead? Just install it on top of the existing install (be sure to shut down Firefox completely before actually running the installer though).

This should take care of the issue, hopefully. :) Please let me know if it does (or doesn't!).

Which security software (firewall, anti-virus) do you have?

If an update fails then it is possible that other (security) software is preventing the updater from replacing the files.

You can try these:

• run the Firefox installer program or Firefox itself as Administrator (right-click: Run as Administrator)
• boot the computer in Windows Safe mode with network support (press F8 on the boot screen)

You may need to delete the Firefox program folder yourself if you keep having problems.

do a clean reinstall and delete the Firefox program folder (C:\Program Files\Mozilla Firefox\) before reinstalling a fresh copy of Firefox. 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.

• http://kb.mozillazine.org/Installation_directory

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 15.0: http://www.mozilla.org/en-US/firefox/all.html

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.
• http://kb.mozillazine.org/Uninstalling_Firefox

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 or if you uninstall Firefox.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

Thanks for your replies! But these didn't help to solve this problem. Accidentally, I found it's all about the wrong setting of cache path. The cache path has been set to H disk, but the ramdisk changed to F disk while there's no H disk anymore.