Pref migration works! (sort of)  See news://news.mozilla.org/37858A46.865119A9%40netscape.com for full details.

Read this posting to figure out where to put your prefs.js file.
If you are using 5.0, that url won't work for you because url dispatching doesn't work yet. Here's the message as a html file.

Address Book Preferences

Here is a sample prefs.js file for 3 accounts: one Pop, one NNTP and one IMAP.
Only change the red dummy values. Leave everything else the same. Make sure you have those first three lines (2 comments and 1 new line).

For Windows:

// Mozilla User Preferences
// This is a generated file! Do not edit.

user_pref("mail.accountmanager.accounts", "account1,account2,account3");
user_pref("mail.account.account1.server", "server1");
user_pref("mail.account.account1.identities", "id1");
user_pref("mail.account.account2.server", "server2");
user_pref("mail.account.account2.identities", "id1");
user_pref("mail.account.account3.server", "server3");
user_pref("mail.account.account3.identities", "id1");
user_pref("mail.server.server1.type", "pop3");
user_pref("mail.server.server1.hostname", "dredd");
user_pref("mail.server.server1.userName", "test");
user_pref("mail.server.server1.password", "clear text pop passsword");
user_pref("mail.server.server1.directory", "c:\\program files\\netscape\\users\\test\\mail\\dredd");
user_pref("mail.server.server1.check_new_mail", true);
user_pref("mail.server.server1.check_time", 10);
user_pref("mail.server.server2.type", "nntp");
user_pref("mail.server.server2.hostname", "news.mozilla.org");
user_pref("mail.server.server2.directory", "c:\\program files\\netscape\\users\\test\\news\\host-news.mozilla.org");
user_pref("mail.server.server2.newsrc.file", "c:\\program files\\netscape\\users\\test\\news\\newsJA5I.rc");
user_pref("mail.server.server3.type", "imap");
user_pref("mail.server.server3.hostname", "tintin");
user_pref("mail.server.server3.userName", "test");
user_pref("mail.server.server3.password", "clear text imap passsword");
user_pref("mail.server.server3.directory", "c:\\program files\\netscape\\users\\test\\ImapMail\\tintin");
user_pref("mail.identity.id1.fullName", "Mozilla Q. Dragon");
user_pref("mail.identity.id1.organization", "mozilla.org");
user_pref("mail.identity.id1.useremail", "test@netscape.com");
user_pref("mail.identity.id1.smtp_server", "dredd");
user_pref("mail.identity.id1.smtp_name", "test");
user_pref("news.max_articles",50);

// Mozilla User Preferences
// This is a generated file! Do not edit.

user_pref("mail.accountmanager.accounts", "account1,account2,account3");
user_pref("mail.account.account1.server", "server1");
user_pref("mail.account.account1.identities", "id1");
user_pref("mail.account.account2.server", "server2");
user_pref("mail.account.account2.identities", "id1");
user_pref("mail.account.account3.server", "server3");
user_pref("mail.account.account3.identities", "id1");
user_pref("mail.server.server1.type", "pop3");
user_pref("mail.server.server1.hostname", "dredd");
user_pref("mail.server.server1.userName", "test");
user_pref("mail.server.server1.password", "clear text pop passsword");
user_pref("mail.server.server1.directory", "/disk/test/.mozilla/test/Mail/dredd");
user_pref("mail.server.server1.check_new_mail", true);
user_pref("mail.server.server1.check_time", 10);
user_pref("mail.server.server2.type", "nntp");
user_pref("mail.server.server2.hostname", "news.mozilla.org");
user_pref("mail.server.server2.directory", "/disk/test/.mozilla/test/News/host-news.mozilla.org");
user_pref("mail.server.server2.newsrc.file", "/disk/test/.newsrc-news.mozilla.org");
user_pref("mail.server.server3.type", "imap");
user_pref("mail.server.server3.hostname", "tintin");
user_pref("mail.server.server3.userName", "test");
user_pref("mail.server.server3.password", "clear text imap passsword");
user_pref("mail.server.server3.directory", "/disk/test/.mozilla/test/ImapMail/tintin");
user_pref("mail.identity.id1.fullName", "Mozilla Q. Dragon");
user_pref("mail.identity.id1.organization", "mozilla.org");
user_pref("mail.identity.id1.useremail", "test@netscape.com");
user_pref("mail.identity.id1.smtp_server", "dredd");
user_pref("mail.identity.id1.smtp_name", "test");
user_pref("news.max_articles",50);

• Don't set the "mail.server.server1.directory" pref to your real nsmail directory. Instead, make a copy of your nsmail directory and point the "mail.server.server1.directory" pref to that copy.  Mozilla 5.0 is still alpha software.
• Make sure that the directory you set as your "mail.server.server1.directory" exists. (#4861)
• Folders with a more than 100 messages will take a long time to load the first time.  (If you are using news, make sure to use the "news.max_articles" pref.)

// Mozilla User Preferences
// This is a generated file! Do not edit.

user_pref("mail.accountmanager.accounts", "account1,account2,account3");
user_pref("mail.account.account1.server", "server1");
user_pref("mail.account.account1.identities", "id1");
user_pref("mail.account.account2.server", "server2");
user_pref("mail.account.account2.identities", "id1");
user_pref("mail.account.account3.server", "server3");
user_pref("mail.account.account3.identities", "id1");
user_pref("mail.server.server1.type", "pop3");
user_pref("mail.server.server1.hostname", "dredd");
user_pref("mail.server.server1.userName", "test");
user_pref("mail.server.server1.password", "clear text pop passsword");
user_pref("mail.server.server1.directory", "HD:System Folder:Preferences:Netscape Users:Test:Mail:dredd");
user_pref("mail.server.server1.check_new_mail", true);
user_pref("mail.server.server1.check_time", 10);
user_pref("mail.server.server2.type", "nntp");
user_pref("mail.server.server2.hostname", "news.mozilla.org");
user_pref("mail.server.server2.directory", "HD:System Folder:Preferences:Netscape Users:Test:News:host-news.mozilla.org");
user_pref("mail.server.server2.newsrc.file", "HD:System Folder:Preferences:Netscape Users:Test:News:news.mozilla.org");
user_pref("mail.server.server3.type", "imap");
user_pref("mail.server.server3.hostname", "dredd.mcom.com");
user_pref("mail.server.server3.userName", "test");
user_pref("mail.server.server3.password", "clear text imap passsword");
user_pref("mail.server.server3.directory", "HD:System Folder:Preferences:Netscape Users:Test:ImapMail:dredd.mcom.com");
user_pref("mail.identity.id1.fullName", "Mozilla Q. Dragon");
user_pref("mail.identity.id1.organization", "mozilla.org");
user_pref("mail.identity.id1.useremail", "test@netscape.com");
user_pref("mail.identity.id1.smtp_server", "dredd");
user_pref("mail.identity.id1.smtp_name", "test");
user_pref("news.max_articles",50);

The news prefs have changed.  In order to avoid using a news fat file on Windows and Mac, we now store the location of the fat file in the prefs.

For each nntp server, you need to point to the newsrc file directly, and to the directory where the summary files will be put.

For example:

user_pref("mail.server.server2.type", "nntp");
user_pref("mail.server.server2.hostname", "news.mozilla.org");
user_pref("mail.server.server2.directory", "/disk/test/.mozilla/test/News/host-news.mozilla.org");
user_pref("mail.server.server2.newsrc.file", "/disk/test/.newsrc-news.mozilla.org");

The summary files for the nntp server on news.mozilla.org will be put under /disk/test/.mozilla/test/News/host-news.mozilla.org and the newsrc file lives at /disk/test/.newsrc-news.mozilla.org

To get new news messages, select a newsgroup and click on the "Get Msg" button.

(Getting new news articles on an entire server isn't supported, so if you select a news server and click the "Get Msg" button, nothing will happen.)

You must add the following line to your prefs.js file to make news usable for newsgroups with large numbers of articles:

user_pref("news.max_articles",50);

This will only dowload the last 50 new news messages. There are performance problems that prevent us from having thousands of messages in the thread pane.

Multiple news servers are supported.  All you need to do is create a new account and server (and identity, but that's not required) in your prefs.js file for each news server.
Here's a sample prefs.js for Linux for three news servers:  news.mozilla.org, news.foo.com, and news.bar.org.

For each news server, you need to set newsrc.file pref to point to the newsrc file, and the directory pref to point to the directory where the summary files will be kept.

// Mozilla User Preferences
// This is a generated file! Do not edit.

user_pref("mail.accountmanager.accounts", "account1,account2,account3,account4");
user_pref("mail.account.account1.server", "server1");
user_pref("mail.account.account1.identities", "id1");
user_pref("mail.account.account2.server", "server2");
user_pref("mail.account.account2.identities", "id1");
user_pref("mail.account.account3.server", "server2");
user_pref("mail.account.account3.identities", "id1");
user_pref("mail.account.account4.server", "server2");
user_pref("mail.account.account4.identities", "id1");
user_pref("mail.server.server1.type", "pop3");
user_pref("mail.server.server1.hostname", "dredd");
user_pref("mail.server.server1.userName", "test");
user_pref("mail.server.server1.password", "clear text pop passsword");
user_pref("mail.server.server1.directory", "/disk/test/.mozilla/test/Mail/dredd");
user_pref("mail.server.server2.type", "nntp");
user_pref("mail.server.server2.hostname", "news.mozilla.org");
user_pref("mail.server.server2.directory", "/disk/test/.mozilla/test/News/host-news.mozilla.org");
user_pref("mail.server.server2.newsrc.file", "/disk/test/.newsrc-news.mozilla.org");
user_pref("mail.server.server3.type", "nntp");
user_pref("mail.server.server3.hostname", "news.foo.com");
user_pref("mail.server.server3.directory", "/disk/test/.mozilla/test/News/host-news.foo.com");
user_pref("mail.server.server3.newsrc.file", "/disk/test/.newsrc-news.foo.com");
user_pref("mail.server.server4.type", "nntp");
user_pref("mail.server.server4.hostname", "news.bar.org");
user_pref("mail.server.server4.directory", "/disk/test/.mozilla/test/News/host-news.bar.org);
user_pref("mail.server.server4.newsrc.file", "/disk/test/.newsrc-news.bar.org");
user_pref("mail.identity.id1.fullName", "Mozilla Q. Dragon");
user_pref("mail.identity.id1.organization", "mozilla.org");
user_pref("mail.identity.id1.useremail", "test@netscape.com");
user_pref("mail.identity.id1.smtp_server", "dredd");
user_pref("mail.identity.id1.smtp_name", "test");
user_pref("news.max_articles",50);
 

Address Book Preferences

• To see the address books and directories on the Address Book window left pane,  copy the "ldap_2.*" prefs from 4.5 prefs.js into prefs.js.