See:

• How to fix the Update Failed error message when updating Firefox
• 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)

Have tried this update over and over always fails. I let it run after I went to bed and it ran for over 8 hours. In the morning it was in the same spot as when it started. When I opened Firefox a minute ago it tried to start automatically and it stayed in the same loop it is always in. I've had Firefox since 2008 and have loaded all updates with success till this one. I had no problem with 4.0. This is the first time I've seen an update like this one.

I should be working right away. If there are problems and it last longer than expected then you can download the full version of that next version (update) and install that 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
• Firefox 3.6.x: http://www.mozilla.com/en-US/firefox/all-older.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.

Your bookmarks and other profile data are stored elsewhere (not in the Firefox program 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.