Netscape occasionally gets a new version number, based on a relatively old version of Mozilla. Netscape also contains AOL bloat.
Mozilla contains a browser, email client, newsreader, IRC client and HTML editor.
Mozilla Firefox is just a web browser, using the HTML rendering codebase of Mozilla. It has some features Mozilla's browser hasn't, and vice versa.
Mozilla Thunderbird is just an email client, based on the email client from Mozilla. Mozilla's email client has been effectively discontinued, all the mailnews dev (pretty much) goes into Thunderbird now. Thunderbird has many more features, and Moz mailnews is desperately lacking in places.
I use Firefox 0.9.3 and Thunderbird 0.7.3. IMO they are very nice.