I think the latest version of mozilla is 1.2.1 that you can get thru apt-get/synaptic. I dont think they've updated that package yet... if they will ever do.
Anyway, you can get mozilla 1.4 at mozilla's homepage. The installation is simple. Should not be any problems to install it.
By default, it would install at /usr/local/mozilla. That would not overwrite Mozilla 1.2 which is located at /usr/lib/mozilla-1.2.1. So, if you want to overwrite the previous mozilla, simply install at /usr/lib, instead of /usr/local... I think...