So i gave him advice to remove the Mozilla's preference. The disadvantage of this method is that your customization will be gone, since it's a directory which saved all of your settings, including your extensions and themes. In Windows it's located in C:\Documents and Settings\UserName\Application Data\Mozilla and C:\Documents and Settings\UserName\Local Settings\Application Data\Mozilla and in Linux, it's located on your home directory and inside a directory called .firefox. Firefox will create this directory automatically if it doesn't exist, so it's a preferred way if you have problems with Firefox. After deleting this directories, he could start his Firefox and work normally.
Another problem was that he never save his extensions or themes, so he will have to search and download again. Well... happy hunting....