Read this posting to figure out where to put your prefs50.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.

Here is a sample prefs50.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");
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");
user_pref("mail.server.server3.type", "imap");
user_pref("mail.server.server3.hostname", "dredd");
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");
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/mozillamail");
user_pref("mail.server.server2.type", "nntp");
user_pref("mail.server.server2.hostname", "news.mozilla.org");
user_pref("mail.server.server2.directory", "/disk/test/mozillanews");
user_pref("mail.server.server3.type", "imap");
user_pref("mail.server.server3.hostname", "dredd");
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/mozillaImapMail");
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");
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");
user_pref("mail.server.server3.type", "imap");
user_pref("mail.server.server3.hostname", "dredd");
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");
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);

To get new news messages, select a newsgroup and click on the "Get Msg" button.  Getting new news articles on an entire server doesn't work yet.

You must add the following line to your prefs50.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 message. 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 prefs50.js file for each news server.  Here's a sample prefs50.js for Linux for three news servers:  news.mozilla.org, news.foo.com, and news.bar.org.  In this example, the newsrc files would be /disk/test/mozillanews/.newsrc-news.mozilla.org, /disk/test/mozillanews/.newsrc-news.foo.com and /disk/test/mozillanews/.newsrc-news.bar.org

// 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/mozillamail");
user_pref("mail.server.server2.type", "nntp");
user_pref("mail.server.server2.hostname", "news.mozilla.org");
user_pref("mail.server.server2.directory", "/disk/test/mozillanews");
user_pref("mail.server.server3.type", "nntp");
user_pref("mail.server.server3.hostname", "news.foo.com");
user_pref("mail.server.server3.directory", "/disk/test/mozillanews");
user_pref("mail.server.server4.type", "nntp");
user_pref("mail.server.server4.hostname", "news.bar.org");
user_pref("mail.server.server4.directory", "/disk/test/mozillanews");
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);
 

On UNIX,  your "mail.server.*.directory"prefs need to point to a directory that contains the .newsrc-<hostname> files.

On Windows, your "mail.server.*.directory"prefs need to point to a directory that contains the"fat" file (from 4.x).  If you choose to copy your News directory (from 4.x) and point your prefs50.js to that new directory, you will need to hand edit your "fat".  (In 4.x, the "fat" file contained absolute paths to the newsrc files.)

On Mac, your "mail.server.*.directory"prefs need to point to a directory that contains the "NewsFAT" file (from 4.x).  If you choose to copy your News directory (from 4.x) and point your prefs50.js to that new directory, you won't need to edit your "NewsFAT" file.  (In 4.x, the "fat" file contained relative paths to the newsrc files.)