Most end user questions for Mozilla mailnews apply to Netscape 7.x, so they will be covered by the Netscape Unofficial FAQ or Netscape Help
There are some differences between Mozilla 1.x and Netscape 7.x, so this FAQ will cover some of those questions.
Q: Why didn't Mozilla migrate my Netscape 4.x address book? Can Mozilla import my Netscape 4.7 address book?
A: Mozilla can't migrate because the code to read the 4.x format is not open source.
The work around is to use 4.7 to export the addressbook to LDIF, and then import it into mozilla.
See http://groups.google.com/groups?selm=t1igrudjto8j11%40corp.supernews.com&oe=UTF-8&output=gplain for instructions.
Note, Netscape 7.x can migrate your Netscape 4.x address book (when you migrate your 4.x profile.)
It can also import Netscape 4.x address books. (From the address book window, Tools | Import...)
Q: Why doesn't Mozilla have a spell checker?
A: The spell checker code that Netscape 7.x uses isn't open source.
For Mozilla, see http://spellchecker.mozdev.org/
Note, Netscape 7.x comes with a spell checker.