Move to a new PC
Mozilla applications don't store your data in the program directory. They store it in a "profile" directory instead in order to make it easier to upgrade or re-install without worrying about losing your data. For example, if you're using Thunderbird your profile has all of your mail, address books, settings, saved passwords, add-ons etc.
The simplest solution if you're using Windows would be to use Mozbackup to backup the complete profile, copy the *.pcv file it creates to the new PC, install the Mozilla application and Mozbackup on the new PC, and then use Mozbackup to restore the profile. Don't delete anything on your old PC until you've verified you can use the moved profile.
If you have problems restoring the profile see the Mozbackup article. Mozbackup is extremely popular and usually works without a problem, but due to its popularity there is plenty of experience in how to deal with problems.
Mozilla applications such as Thunderbird, Firefox and SeaMonkey use a Profiles.ini file to find profiles. So just copying a profile to the default location won't work. You either need to use an application that updates profiles.ini for you like Mozbackup, edit it yourself, or copy the profile over an existing one that is already listed in profiles.ini. One advantage of it using profiles.ini is that you can store the profile almost anywhere (just don't store it in your applications program directory).
Not using Windows
If you're not using Windows see Moving your profile for another solution. One quick and dirty solution would be to start the Mozilla application on the new PC and immediately exit. That will create a stub of a profile (if you didn't have a profile on that PC). You could then find its location and copy the contents of your old profile over it. That is described in more detail here . However, we've learned from helping people in the forums that there are a lot less problems if you use a not well known feature of the profile manager to tell the application where the profile is instead. That is described here.
PC died
If you are moving to a new PC because your PC died, if the hard drive still works you can attach it to the new PC and then move the profile per one of the methods above. If you are using Windows the main problem will be finding where its stored because just typing in %APPDATA% in Start → Run won't tell you the profiles location anymore because that uses the profiles.ini file for your current Windows account, not the one on the old hard disk. You will need to remember your old Windows username and browse the C:\Documents and Settings\<old windows username>\Application Data directory tree to find it. See the links in "See also" (below) for an article on how to find your profile. The exact location depends upon both what application and what operating system you're using.
