Recent versions of Netscape Communicator are built on top of Mozilla (Thunderbird), so both e-mail clients share the same folder locations for e-mail database:

C:\Documents and Settings\User Name\Application Data\Mozilla\Profiles\
C:\Windows\Profiles\User Name\Application Data\Mozilla\Profiles\
C:\Windows\Application Data\Mozilla\Profiles\


Mozilla profile contains your bookmarks, mail/news files and account settings, stored passwords, address book, security certificates and preferences.
Mail. Mail messages are stored in the Mail sub-directory. To back up your mail messages, save the files without extension, for example INBOX. To back up filters too, save the rules.dat file. The *.msf files are index files that will be automatically regenerated if removed. Save the entire Mail directory if you are uncertain what to save.
News. News settings and messages are stored in the News sub-directory. Your subscribed groups are stored in the *.rc files. The complete news server groups list is stored in hostinfo.dat. *.msf files store message summaries. To back up news, save the *.rc, *.dat, and *.msf files.
Address book. In your profile directory, save abook.mab (personal address book) and history.mab (collected addresses). If you have more than one address book, save all *.mab files.
Bookmarks. Save the bookmarks.html file.
Saved password data. Username and password data is stored in 12345678.s (the number part is random). Save the file.
Junk filters. Save the training.dat file which stores information for the new smart filtering system.
Cookies. Save the cookies.txt file.