Ok I have apt-get and when i try to run $ apt-get upgrade mozilla it tells me that I have the most current version.
Did you run $apt-get update before trying to run $apt-get install mozilla (I think "install" is the correct command, "upgrade" is used for upgrading your Linux version?
Check your etc/apt/sources.list
You should have something like this at least: (or a Japanese mirror : ftp.jp.debian.org ....)
woody main contrib non-free
sarge main contrib non-free
sid main contrib non-free
Check the mozilla version you want on the debian site (packages : the 1.7 version may be in sarge (testing) or in sid (unstable).
I don't bother about debian packages when it comes to Mozilla and its consorts (Firefox, thunderbird): I download the tar.gz files from the mozilla web site and install it (very easy, since they are binary files) -- this way I'm sure to get the latest stable version.
As I had Mozilla 1.0 on my Linux distro, i upgraded to the latest woody version (1.5 at the time) to make sure I got all the necessary libraries installed on the system. I didn't remove that Mozilla version (i thought i might remove the libraries as well), and installed Mozilla from the tar.gz file (the web wite tells you how to install). It installs into a diffrent dir (/usr/local/ is usually the one) from the deb package, so there shouldn't be any problems.
As for plugins, yours may be outdated, especially the java, and flash plugins (there's a whole page on the java site telling you how to install java and the plugin).
If you remove Mozilla, the plugins folder will go too. Make a copy of it in your /home/dir so next time you only have to copy this folder into your mozilla program dir.