Move to a new PC
From MozillaZine Knowledge Base
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.
Use Mozbackup 1.5.2 Beta 1 (there is a download link for it in the News section) rather than the version available at the normal download link. Mozbackup 1.5.1 usually works fine but a few users have had problems where it doesn't backup everything it is supposed to. The beta doesn't seem to have that problem.
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. If you can't find the Mozbackup download and you're using Firefox, look in Tools -> Options -> General ->Downloads to see where it downloads files by default.
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.
Copy and paste just a few files instead
This explains how to copy the personal address book and the mail folders for a single POP account to the new PC if for some reason you don't want to copy the entire profile. This is actually more work than using Mozbackup but might be easier for some people since it doesn't have any external dependencies and all of the instructions are in one place.
1. Start Thunderbird on your old PC.
2. Press the Address Book button.
3. Select your personal address book, and use Tools -> Export to export it as personal.ldif (a .LDIF file).
4. Copy personal.ldif to a storage location such as a USB flash drive.
5. Find your account in Tools -> Account Settings. Select Server Settings. Look at the bottom of the pane for "Local Directory", next to a browse button. That specifies where the mail folders (they're stored as mbox files) are stored for your account. They are named after the folder, and have no file extension. For example your inbox is "inbox.", not "inbox.msf" or "inbox.sbd". If you're using a global inbox look in the Local Folders pane instead.
6. Use Windows Explorer (or My Computer) to go to that directory.
7. Copy the mail folders you want to a storage location such as a USB flash drive.
8. Start Thunderbird on your new PC.
9. If you don't have an account let it run the account wizard, and create your account.
10. Use Tools -> Import -> Address Books -> Text Files to import personal.ldif. That will create a new address book. Drag and drop its contents to the personal address book and then delete the imported address book.
11. Copy the mbox files either to the accounts Local Directory or Local Folder's Local Directory. You can find their location the same way (Tools -> Account Settings, and find the Local Directory either in the accounts Server Settings pane or in the Local Folders pane).
Its easy to modify this if you have more than one POP account. If you have a IMAP account the messages should still be in remote folders on the IMAP server, so there is no need to copy them.
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 ..\Documents and Settings\<old windows username>\Application Data directory tree on your old hard disk 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.
Moving from Windows to OSX or Linux
The profile contains a prefs.js file that stores both absolute and relative path names for where each account stores its mail. A relative path name means you don't specify the full path, you specify its location relative to where the profiles.ini file is stored.
If you move a profile between two different versions of Windows the syntax is identical. However, if you move it between dissimilar operating systems the syntax is incompatible and may break the absolute version of a path. For example, Linux has no concept of a drive C:. Normally this is harmless since Thunderbird prefers to use the relative version. However, if this causes a problem (most of the folders for an account aren't visible, and your inbox looks empty) find the local directory setting at the bottom of the Account Settings page, and use the "Browse" button to update its location. Normally it points to a directory named after the mail server used by that account, unless you using a global inbox. In that case it points to the Local Folders directory in the profile.